單項(xiàng)選擇題

The crank-room cover should not be opened until at least （） minutes after the engine stops.

A.10
B.15
C.20
D.30

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單項(xiàng)選擇題

The crankcase cover should never be allowed to open for investigation when （）

A.the engine stops
B.the engine is running
C.you repair the engine
D.overhauling the engine

單項(xiàng)選擇題

The crankcase can not be opened （）the parts have been cooled down.

A.if
B.as
C.unless
D.when