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
Genetics is the study of the biological factors that influence heredity and determine variation in living things.
A．duplicate
B．affect
C．clarify
D．improve
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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