单项选择题

Even before modern notions of friction, it was known that greased surfaces move over one another more easily than dry ones.

A．concepts
B．tricks
C．benefits
D．techniques
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单项选择题
Cactus plants have spines that prevent animals from nibbling them.
A．ruining
B．reaching
C．climbing
D．gnawing
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
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