单项选择题

The atmosphere, a narrow band of gases surrounding the Earth, is maintained by gravitational pull.

A. clear
B. freezing
C. thin
D. poisonous
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单项选择题
An oyster produces a pearl by coating a grain of sand inside its shell with nacre, a secretion of its body.
A. makes
B. attains
C. binds
D. cures
单项选择题
Evolution has hardly changed the horseshoe crab over millions of years.
A. carefully
B. probably
C. scarcely
D. slowly
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