根据《水利工程设计概（估）算编制规定》，施工企业进退场补贴费应在（）中列支。

The reasons why birds don't get lost on long flights ______.

A．have been known to scientists for years

B．have only recently been discovered

C．are known by everyone

D．will probably remain a mystery

Task 2

Directions: This task is the same as Task 1. The 5 questions or unfinished statements are numbered 41 through 45.

Why don't birds get lost on their long flights from one place to another？ Scientists have puzzled over this question for many years. Now they are beginning to fill in the blanks.

Not long ago, experiments showed that birds depend on the sun to guide

The reason why birds don't get lost on long flight ______.

A．has been discovered recently

B．is known by everybody

C．still remains a mystery

D．has been known to scientists for years

Task 1

Directions: After reading the following passage, you will find 5 questions or unfinished statements, numbered 36 through 40. For each question or statement there are 4 choices marked A, B, C, and D. You should make the correct choice.

Why don't birds get lost on their long flights from one place to another？ Scientists have puzzled over this question for many years. Now they're beginning to fill in the blanks.

Not long ago, experiments showed that birds rely on the sun to guide th

The reason why birds don't get lost on long flights ______.

A．have been known to scientists for years

B．have only recently been discovered

C．are known by everyone

D．will probably remain a mystery

听力原文： Why don't birds get lost on their long flights from one place to another？ Scientists have puzzled over this question for many years. Now they are beginning to fill in the blanks.

Not long ago, experiments showed that birds rely on the sun to guide them during daylight hours. But what about birds that fly by night？ Tests with artificial stars have proved that certain night-flying birds are able to follow the stars in their long-distance flights.

A dove had spent his lifetime in a cage and had never flown under a natural sky. Yet it showed an inborn ability to use the stars for guidance. The bird's cage was placed under an artificial star-filled sky. The bird tried to fly in the same direction as that taken by his outdoor cousins. Any change in the position of the artificial stars caused a change in the direction of his flight.

But the stars are apparently their principal means of navigation. When the stars are hidden by clouds, they apparently find their way by such landmarks as mountain ranges, coast lines, and river courses. But when it's too dark to see these, the doves circle helplessly, unable to find their way.

The reason why birds don't get lost on long flights ______.

A．has been known to scientists for years

B．has only recently been discovered

C．is known by everyone

D．will probably remain a mystery

puzzled over this question for many years. Now they're beginning to fill in the blanks.

Not long age, experiments showed that birds rely on the sun to guide them during daylight hours. But what about birds that fly by night？ Tests with artificial stars have proved that certain night - flying birds are able to follow the stars in their long - distance flights.

A dove (鸽子) had spent its lifetime in a cage and had never flown under a natural sky. Yet it showed an inborn ability to use the stars for guidance. The bird's cage was placed under an artificial star - filled sky. (76) The bird tried to fly in the same direction as that taken by his outdoor cousins. Any change in the position of the artificial stars caused a change in the direction of his flight.

(77) But the stars are apparently their principal means of navigation (航行) only. When the stars are hidden by clouds, they seemingly find their way by such landmarks as mountain ranges, coast lines, and river courses. But when it's too dark to see these, the doves circle helplessly, unable to find their way.

The reason why birds don't get lost on long flights ______.

A．have been known to scientists for many years

B．have only recently been discovered

C．are known by us

D．will probably remain a mystery

been puzzled over this question for many years. Now they're beginning to fill in the blanks.

Not long ago, experiments showed that birds rely on the sun to guide them during daylight hours. But what about birds that fly by night？ (37) Tests with artificial stars have proved that certain night-flying birds are able to follow the stars in their long-distance flights.

A dove had spent its lifetime in a cage and had never flown under a natural sky. Yet it showed an inborn ability to use the stars for guidance. The bird's cage was placed under an artificial star-filled sky. The bird tried to fly in the same direction as that taken by his outdoor cousins. Any change in the position of the artificial stars caused a change in the direction of his flight.

The stars are apparently their principal means of navigation. When the stars are hidden by clouds, they seemingly find their way by such landmarks as mountain ranges, coast lines, and river courses. But when it's too dark to see these, the doves circle helplessly, unable to find their way.

The reason why birds don't get lost on long flights______.

A．has been known to scientists for years

B．has only recently been discovered

C．is known by everyone

D．will probably remain a mystery

puzzled over this question for many years. Now they are beginning to fill in the blanks.

Not long ago, experiments showed that birds depend on the sun to guide them during day-flight hours. But what about birds that fly by night？ Tests with man-made stars have proved that certain night-flying birds are able to follow the stars in their long-distance flights.

A dove spent its lifetime in a cage and had never flown under a natural sky. Yet it showed an inborn ability to use the stars for guidance. The bird's cage was placed under a man-made star-filled sky. The bird tried to fly in the same direction as that taken by his outdoor cousins. Any change in the position of the make-believe stars caused a change in the direction of his flight.

Scientists think that doves, flying ill daylight, use the sun for guidance. But the stars are obviously their most important means of navigation (or flying). What do they do when the stars are hidden by clouds？ Obviously, they find their way by such landmarks as mountains ranges, coastlines, and river courses. But when it is too dark to see these, the doves circle helplessly, unable to get their bearings.

The reason why birds don't get lost on long flight ______.

A．has been discovered recently

B．is known by everybody

C．still remains a mystery

D．has been known to scientists for years