阅读理解

　　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(依赖) on the sun to guide them during daylight hours. But what about birds that fly at night? Tests with artificial(人造的) stars have proved that certain night-flying birds are able to follow the stars on 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 birds's cage was placed under an artificial starfilled 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, when flying in daylight, use the sun for guidance. But the stars are obviously their principal(主要的) 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 mountain ranges, coastlines, and river courses. But when it's too dark to see these, the doves circle helplessly, unable to get their bearings(方位).

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

[　　]

A．is known by everybody

B．still remains a mystery

C．has been discovered recently

D．has been known to scientists for years

2．Experiments showed that ________.

[　　]

A．birds rely on the sun to guide them

B．birds rely on the stars to guide them

C．birds are likely to get lost if there aren't artificial stars

D．day-flying birds rely on the sun while night-flying birds rely on the stars

3．The experiment on the dove indicated(表明) that ________.

[　　]

A．a bird that has been caged will not fly long distances

B．some birds seem to follow the stars when they fly at night

C．a bird having spent its lifetime in a cage has to- be taught how to navigate

D．birds can fly in the same direction as that taken by their cousins

4．Under artificial stars, the bird in the cage ________.

[　　]

A．tried to fly in the same direction as birds not caged

B．changed direction when the position of the stars was changed

C．would not fly well

D．both A and B

5．In total darkness, doves ________.

[　　]

A．circle helplessly

B．use landmarks

C．find their way by clouds

D．fly back home

Lucy, would you please turn on the light? It's too dark to see anything ________（清楚的）.