单项选择题

Reserve buoyancy is the ().
A.Unoccupied space below the waterline
B.Volume of intact space above the waterline
C.Excess of the buoyant force over the gravity force
D.Difference in the buoyant force in salt and fresh waters

A.
A.Unoccupied
B.Volume
C.Excess
D.Difference

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B.所间区间
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The exhaust ports of a diesel engine using the cross-flow scavenging method are opened and closed by the _________.
A.reciprocating motion of exhaust valves
B.rotary motion of the camshaft
C.reciprocating motion of the piston
D.developed differential

A.
A.reciprocating
B.rotary
C.reciprocating
D.developed