A drop in compression pressure in one cylinder of a diesel engine can be caused by（）

A.on the power stroke
B.on the exhaust stroke
C.at the end of the power stroke
D.at the start of the intake stroke

A.early fuel injection timing
B.leaking valves
C.a continuously open scavenge air port
D.low cooling water temperature

A.clogged air filter
B.compression stroke
C.burned exhaust valves
D.all of the above