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單項選擇題

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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單項選擇題

These piston rings are so badly worn that they must be （）.

A.renewed
B.welded
C.machined
D.chromium-plated

單項選擇題

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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