For a diesel engine, when the piston is at the （）, the clearance volume is the largest.

A.the cylinder liner
B.the cylinder cover
C.the cylinder jacket
D.the piston

A.exhaust valves
B.mechanical lubricators
C.cooling water pumps
D.fuel injection pumps

A.throughput
B.back pressure
C.gravity disc
D.None of the above is wrong