单项选择题

What is the main topic of the passage

A．The ways in which crows differ from other common birds.
B．The myths and legends about crows.
C．The characteristics that make crows difficult to study.
D．The existing methods for investigating crow behavior.

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单项选择题

According to the third paragraph, which of the following is true about crows

A．They seldom live in any one place or very long.
B．They thrive in a wide variety of environments.
C．They have marked preferences for certain kinds of foods.
D．They use up the resources in one area before moving to another.

单项选择题

According to the second paragraph, crows are poor subjects for field research for all the following reasons EXCEPT ______.

A．they can successfully avoid observers
B．they are hard to distinguish from one another
C．they can be quite aggressive
D．they are difficult to catch