单项选择题

Enamel,the hardest substance in a human being's body,covers the entire crown of the tooth.

A．whole
B．inseparable
C．superficial
D．main
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单项选择题
There are notices to the contrary,a great deal of technical writing is at best awkward and at worst actually unclear.
A．denials
B．laws
C．attempts
D．ideas
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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