问题描述：
[单选]
The lowest ring of a piston is（）．
A．a coil spring
B．a sealing ring
C．an oil distributor ring
D．main seal ring
参考答案：查看无
答案解析：无
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答案解析：无
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上一篇：The lubricating oil is pumped from the drain tank， very often with（）in the suction line．
下一篇：The lower end of each piston rod is reduced in diameter to fit a bore in（）．
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