问题描述：
[单选]
There should be a lubricating oil（）between the main beatings and the crankshaft．
A．film
B．cap
C．lacquer
D．shell
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答案解析：无
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上一篇：These piston rings are so badly worn that they must be（）．
下一篇：There is an injection pump for each cylinder， which is usually operated once every cycle by（）on the camshaft．
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