单项选择题
Pistons may be made in （） in order to keep the weight down for balancing purposes at high speed.
A.cast iron B.forged steel C.stainless steel D.aluminum alloy
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（）.
单项选择题 Operating conditions in the crank chamber are very （） those in the cylinder.
