单项选择题
Pistons of marine medium speed diesel engines may be made of （） in order to keep the weight down for balancing purpose at high speed.
A.lead B.forged steel C.alloy steel D.aluminum alloy
单项选择题 Pistons may be made in （） in order to keep the weight down for balancing purposes at high speed.
单项选择题 Piston No2 of main engine has been removed, and it will be （） by noon tomorrow.
单项选择题 Persistent knocking in one cylinder of a diesel engine could be caused by（）.
