单选题In slow speed two-stroke engines,（）the fresh air entering begins when and stops as（）Athe inlet port is opened by the up coming piston； the port is closed by the down goingBthe inlet valve is opened by the down going piston；the inlet valve is closedCthe inlet port is opened by the down going piston；the port is closed by the up comingDthe inlet valve is opened
单选题
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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