If the firing pressures in a diesel engine are high, although the exhaust temperatures are normal, the cause may be（）

A.during the valve overlap period
B.with only the exhaust valve open
C.at a pressure below atmospheric
D.without cooling the cylinders or pistons

A.excessive piston to liner clearance
B.excessive dirt beneath the piston crown that reduces heat transfer
C.faulty nozzle spray
D.all of the above

A.an increased flash point
B.higher lube oil pressures
C.an increased viscosity
D.lower lube oil pressures