Different stimuli can produce the same sensations, and the same stimuli can produce very different sensations.（）

A．To show the students a good time.

B．To introduce the students to the woman.

C．To show the students key buildings on the campus.

D．To make sure that the students don't have any problems.

A．To stress the role of dirt.

B．To introduce the history of dirt.

C．To call attention to the danger of dirt.

D．To present the change of views on dirt.

Why does the professor mention Sirius B at the beginning of his lecture?

A．To introduce the topic of his lecture on white dwarfs.

B．To give a visual example of what he is discussing.

C．To explain the relation between stars and their companions.

D．To compare and contrast two stages in the formation of stars.

Why does the professor mention Sirius B at the beginning of his lecture?

A．To introduce the topic of his lecture on white dwarfs.

B．To give a visual example of what he is discussing.

C．To explain the relation between stars and their companions.

D．To compare and contrast two stages in the formation of stars.

听力原文：M：Let's ask Dan to introduce the musicians to the audience at the beginning of the concert.

W：Ask Dan? He'll be playing the violin!

Q：What does the woman mean?

(15)

A．Dan isn't a very good violinist.

B．Someone else should make the introduction.

C．There will be other musicians to introduce.

D．It's rather late to ask Dan to make the introduction now.

What is the main purpose of the speaker?

A．To introduce the Indian people who have tattoos.

B．To tell us how to get a tattoo.

C．To introduce the history and function of tattoos.

D．To show us how a tattoo influences the wearers in many ways.

How do fireworks produce their colors?

Chemicals that give off bright, distinct colors when burned provide the spectacular colors associated with fireworks. Charcoal and iron, for instance, burn brilliant orange. Strontium salts produce red and barium nitrate gives off green. Blue is the most difficult color to produce, and chemists are still searching for a compound that produces a true shade of blue.

Once a manufacturer chooses the colors for a firework, the appropriate chemical powders are compressed into small pellets called stars. These stars are normally packed into a Shell containing gun powder. The shell is then put in a launcher — a close-fitting tube, closed at one end, that has been hammered into the ground. The tube acts like a cannon as the pressure from the exploding gunpowder and expanding gases become trapped behind the shell arid shoot it into the sky.

Depending on the shell design and amount of stars, the firework display varies. A Roman candle, for example, contains only a few stars and shoots them out one at a time, while an aerial shell contains hundreds of stars — each of which leaves a single trail of brilliant color in the sky. For more information about fireworks, contact the National Council on Fireworks Safety, in Washington, D.C., at www. fireworksafety. Com.

The "stars" described in the passage are______.

A．shells that are put in a launcher

B．heavenly bodies in the sky

C．brilliant colors produced by fireworks

D．compressed chemical powder

The grandest of these ideals is an unfolding American promise that everyone belongs, that everyone deserves a chance that no insignificant person was ever born. Americans are called to enact this promise in our lives and in our laws. And though our nation has sometimes halted, and sometimes delayed, we must follow no other course.