单项选择题

It is necessary to （） cool the various components of the machinery so that the engine can work properly.

A.continuously
B.regularly
C.constantly
D.period

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单项选择题

A.to change the lube oil
B.have the lube oil purified
C.have the lube oil burnt
D.to analyze the lube oil

单项选择题

A.not less than twice
B.greater at least twice than
C.not greater twice than
D.larger three times than

单项选择题

A.not be changed
B.be changed
C.should not be changed
D.should be changed