In some modern large diesel engines, which of the following is used as the support for the main bearings?（）

A.clearance between the top of the valve stem and the rocker arm
B.compression of the valve springs
C.clearance between the valve seat inserts and the head
D.out of roundness of the fuel injection cams

A.low lube oil temperature
B.low lube oil pressure
C.high exhaust temperature
D.high cooling water pressure

A.Dirty lube oil filter
B.Too much piston ring wear
C.High lube oil viscosity
D.Low lube oil temperature