单项选择题

Experiments showed that ______.

A．birds are likely to get lost if there aren't man-made stars.
B．day-flying birds depend on the sun while night-flying birds depend on the stars
C．birds depend on the sun to guide them
D．birds depend on the stars to guide them

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单项选择题

Experiments showed that ______.

A．birds are likely to get lost if there aren't man-made stars.
B．day-flying birds depend on the sun while night-flying birds depend on the stars
C．birds depend on the sun to guide them
D．birds depend on the stars to guide them

单项选择题

The experiment with the dove indicated that ______.

A．birds can fly in the same direction as that taken by their cousins
B．a bird that has been caged will not fly long distances
C．some birds seem to follow the stars when they fly at night
D．a bird having spent its lifetime in a cage has to be taught how to navigate

单项选择题

In total darkness, doves ______.

A．circle helplessly
B．use landmarks
C．find their way by clouds
D．fly back home

单项选择题

Under man-made stars, the bird in the cage ______.

A．tried to fly in the opposite direction of birds not caged
B．changed direction when the position of the stars was changed
C．wouldn't fly well
D．stayed where they were

单项选择题

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