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单选题
The piston moves up and down, thus causing the rotational movement of the（）
A
crankshaft
B
connecting rod
C
cross-head
D
main beating
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reduction gearsC
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the piston nearbyC
the other pistonsD
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the inlet port is opened by the up coming piston； the port is closed by the down goingB
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单选题In a four-stroke engines the camshaft rotates at （）.A
half the engine rotational speedB
twice the engine rotational speedC
the engine rotational speedD
four times the engine rotational speed
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