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
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单项选择题
Evolution has hardly changed the horseshoe crab over millions of years.
A. carefully
B. probably
C. scarcely
D. slowly
单项选择题
Seasonal variations in tides are due mainly to the gravitational pull of the moon.
A. merely
B. mostly
C. likewise
D. somewhat
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