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單項選擇題

In slow speed two-stroke engines,()the fresh air entering begins when and stops as()

A.the inlet port is opened by the up coming piston; the port is closed by the down going
B.the inlet valve is opened by the down going piston;the inlet valve is closed
C.the inlet port is opened by the down going piston;the port is closed by the up coming
D.the inlet valve is opened

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