The intake ports of a two-strike/cycle diesel engine are opened and closed by the action of the （）

A.exhaust pressure
B.valve spring pressure
C.rocker arm movement
D.wrist pin movement

A.piston rod guides
B.engine thrust bearing
C.crosshead bearing
D.main shaft flexible coupling

A.opening the exhaust ports
B.placed on top of the engine along its centerline
C.farthest from the cylinder head
D.nearest to the cylinder head