重力场中，储存能量为In a gravitational field, the stored energy is

The author provides information that answers which of the following questions? Ⅰ. How does the small area's increasing density affect its gravitational field? Ⅱ. What causes the disturbance that changes the cloud from its original static state? Ⅲ. What is the end result of the gradually increas-ing concentration of the small area of gas?

A．Ⅰ only

B．Ⅱ only

C．Ⅰ and Ⅱ only

D．Ⅰ and Ⅲ only

E．Ⅰ, Ⅱ and Ⅲ

The author provides information that answers which of the following questions? Ⅰ. How does the small area's increasing density affect its gravitational field? Ⅱ. What causes the disturbance that changes the cloud from its original static state? Ⅲ. What is the end result of the gradually increas-ing concentration of the small area of gas?

A．Ⅰ only

B．Ⅱ only

C．Ⅰ and Ⅱ only

D．Ⅰ and Ⅲ only

E．Ⅰ, Ⅱ and Ⅲ

The gravitational pull of the Earth and the Moon is important to us as we attempt to conquer more and more of outer-space.Heres why. As a rocket leaves the Earth, the pull of the Earth on it becomes less and less as the rocket roars out into space.If you imagine a line between the Earth and the Moon, there is a point somewhere along that line, nearer to the Moon than to the Earth, at which the gravitation pull of both the Earth and the Moon on an object is just about equal.An object placed on the Moon side of that point would be drawn to the Moon.An object placed on the .Earth side of that point would be drawn to the Earth.Therefore, a rocket need be sent only to this "point of no return" in order to get it to the Moon.The Moons gravity will pull it the rest of the way. The return trip of the rocket to the Earth is, in some ways, less of a problem.The Earths gravitational field reaches far closer to the Moon than does the Moons to the Earth.Thus it will be necessary to fire an Earthbound rocket only a few thousand miles away from the Moon to reach a point where the rocket will drift to the Earth under the Earths gravitational pull. The problem of rocket travel is not so much concerned with getting the rocket into space as it is with guiding the rocket after it leaves the Earths surface.Remember that the Moon is constantly circling the Earth.A rocket fired at the Moon and continuing in the direction in which it was fired would miss the Moon by a wide margin and perhaps continue to drift out into space until "captured" in another planets gravitational field.To reach the Moon, a rocket must be fired toward the point where the Moon will be when the rocket has traveled the required distance.This requires precise calculations of the speed and direction of the rocket and of the speed and direction of the Moon. For a rocket to arrive at a point where the Moons gravity will pull it the rest of the way, it must reach a speed called velocity of escape.This speed is about 25,000 miles per hour.At a speed less than this, a rocket will merely circle the Earth in an orbit and eventually fall back to the Earth.

This passage deals mainly with______.

A．the gravitational pull of the Earth and the Moon

B．the factors involved in firing a rocket into the outer-space

C．the gravitational fields of the Earth and the Moon

D．the speed and direction of a rocket traveling in the outer-space

The gravitational pull of the Earth and moon is important to us as we attempt to conquer more and more of outer-space. Here's why.

As a rocket leaves the Earth, the pull of the Earth on it becomes less and less as the rocket roars out into space. If you imagine a line between the Earth the pull of the Earth and the moon, there is a point somewhere along that line, nearer to the moon than to the Earth, at which the gravitation pull of both the Earth and the moon on an object is just about equal. An object placed on the moon side of that point would be drawn to the moon. An object placed on the Earth side of that point would be drawn to the Earth. Therefore, a rocket need be sent only to this "point of no return" in order to get it to the moon. The moon's gravity will pull it the rest of the way.

The return trip of the rocket to Earth is, in some ways, less of a problem. The Earth's gravitational field reaches far closer to the moon than does the moon's to Earth. Thus it will be necessary to fire an Earthbound rocket only a few thousand miles away from the moon to reach a point where the rocket will drift to earth under the Earth's gravitational pull.

The problem of rocket travel is not so much concerned with getting the rocket into space as it is with guiding the rocket after it leaves the Earth's surface. Remember that the moon is constantly circling the Earth. A rocket fired at the moon and continuing in the direction in which it was fired would miss the moon by a wide margin and perhaps continue to drift out into space until "captured" in another planet's gravitational field. to reach the moon, a rocket must be fired toward the point where the moon will be when the rocket has traveled the required distance. This requires precise calculations of the speed and direction of the rocket and of the speed and direction of the moon.

For a rocket to arrive at a point where the moon's gravity will pull it the rest of the way, it must reach a speed called velocity of escape. This speed is about 25, 000 miles per hour. At a speed less than this, a rocket will merely circle the Earth in an orbit and eventually fall back to Earth.

This passage deals mainly with ______ .

A．the gravitational pull of the Earth and the moon

B．the factors involved in firing a rocket into the outer-space

C．the gravitational fields of the Earth and the moon

D．the speed and direction of a rocket traveling in the outer-space

According to the passage, the most difficult task of fining a rocket is ______.

A．to get the rocket into the space

B．to calculate the rocket's velocity of escape

C．to guide the rocket after it leaves the Earth's surface

D．to identify the gravitational fields of the Earth and the moon

According to the passage,the most difficult task of firing a rocket is____.

A．to get the rocket into the space

B．to calculate the rocket's velocity of escape

C．to guide the rocket after it leaves the Earth's surface

D．to identify the gravitational fields of the Earth and the moon

Part A

Directions: Read the following four texts. Answer the questions below each text by choosing A, B, C or D. (40 points)

NASA launched the first space mission to Pluto yesterday as a powerful rocket hurled the New Horizons spacecraft on a nine-year, three-billion-mile journey to the edge of the solar system

As it soared toward a 2007 meeting with Jupiter, whose powerful gravitational field will shoot it on its way to Pluto. Mission managers said radio communications confirmed that the 1,054-pound craft was in good health.

The $700 million mission began when a Lockheed Martin Atlas 5 rocket rose from a launching pad at the Cape Canaveral Air Force Station in Florida at 2 p.m., almost an hour later than planned because of low clouds that obscured a clear view of the flight path by tracking cameras.

Less than an hour later, all three stages of the booster rocket worked as planned, and the spacecraft separated from them and sprinted away toward deep space. The robot ship sped away at about 36,000 miles per hour, the fastest flight of any spacecraft sent from Earth, allowing it to pass the Moon in about nine hours.

"This is a historic day", said Alan Stem of the Southwest Research Institute in Boulder, Colo, the mission's principal scientist and team leader. Speaking at a news conference at the Kennedy Space Center in Florida, Dr. Stern said the timing assured that the New Horizons would arrive for its closest approach to Pluto on July 14, 2015—the 50th anniversary of the first flyby of Mars by the Mariner 4. the mission that began the exploration of the planets.

The New Horizons is powered by a small plutonium-fired electric generator. Its instruments include three cameras, for visible-light, infrared and ultraviolet images, and three spectrometers to study the composition and temperatures of Pluto's thin atmosphere and surface features. It also carries a University of Colorado dust counter, the first experiment to fly on a planetary mission that is entirely designed and operated by students. This is the only experiment that will not hibernate during the mission.

Yesterday's liftoff also paid regard to Pluto's discoverer, the astronomer Clyde W. Tombaugh, who in 1930 became the only American to find a planet in the solar system.(He died at 90 in 1997.) His widow, Patricia Tombaugh, 93, and other family members were present at the cape, and some of his remains were among the commemorative items aboard the spacecraft. "Some of Clyde's ashes are on their way to Pluto today", Dr. Stem said.

The New Horizons is to reach Jupiter's gravitational field in 13 months. The trip to Pluto will take eight more years, most of which the craft will spend in electronic "hibernation" to save power and wear on the equipment needed for its seven experiments.

In addition to the two-hour delay, the launching was postponed twice in two days—on Tuesday by strong winds at the cape and on Wednesday by a storm that caused a power; failure at the spacecraft's control center at the Johns Hopkins University Applied Physics Laboratory in Laurel. Md. Mission planners had until Feb. 14 to launch the mission this year, but only until the end of this month to use the gravity boost from Jupiter, which will shorten the trip to Pluto by five years.

In the opening paragraph, the author introduces his topic by______.

A．describing a situation

B．justifying an assumption

C．making a comparison

D．presenting a phenomenon

The difference between a liquid and a gas is obvious under the conditions of temperature and pressure commonly found at the surface of the Earth. A liquid can be kept in an open container and fill it to the level of a free surface. A gas forms no free surface but tends to diffuse(扩散)throughout the space available; it must therefore be kept in a closed container or held by a gravitational field, as in the case of a planet's atmosphere. The distinction was a prominent feature of early theories describing the phases of matter. In the nineteenth century, for example, one theory maintained that a liquid could be" dissolved" in a vapor without losing its identity, and another theory that held the two phases are made up of different kinds of molecules. liquids and gases. The theories now prevailing take a quite different approach by emphasizing what liquids and gases have in common. They are both forms of matter that have no permanent structure, and they both flow readily. They are fluids.

The fundamental similarity of liquids and gases becomes clearly apparent when the temperature and pressure are raised somewhat. Suppose a closed container partially filled with a liquid is heated. The liquid expands, or in other words becomes less dense; some of it evaporates. In contrast, the vapor above the liquid surface becomes denser as the evaporated molecules are added to it. The combination of temperature and press at which the densities become equal is called the critical point. Above the critical point the liquid and the gas can no longer be distinguished; there is a single, undifferentiated fluid phase of uniform. density.

Which of the following would be the most appropriate title for the passage?

A．The Properities of Gases and Liquids.

B．What Fluids and Gases Are Made of.

C．New Uses for the Gas and the Liquid.

D．New Theories about Fluids and Gases.

According to the passage, which of the following is an accurate statement concerning meteor streams?

A．Meteor streams and comets start out with smiliar orbits, but only those of meteor streams are perturbed by planetary gravittion.

B．Meteor streams grow as dust particles are attracted by the gravitational fields of comets.

C．Meteor streams are composed of dust particles derived from comets.

D．Comets may be composed of several kinds of materials, while meteor streams consitst only of large dust particles.

E．Once formed, meteor streams hasten the further disintegration of comets.