If the jacket water temperature in an auxiliary diesel engine cooling system is lower than

If the jacket water temperature in an auxiliary diesel engine cooling system is lower than normal, the probable cause is .

A.air binding of the engine cooling system

B.a cracked water cooled exhaust manifold

C.blockage in the heat exchanger

D.faulty operation of the thermostat

请帮忙给出正确答案和分析，谢谢！