问题描述：
[单选]
They understand the（）of automatic sludge discharging of the purifierBut they are not good in practice．
A．principle
B．principal
C．cause
D．case
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答案解析：无
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答案解析：无
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上一篇：This used piston had been tested and approved as a spare part by the（）during last annual repair．
下一篇：These piston rings are so badly worn that they must be（）．
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