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.cast iron
B.forged steel
C.stainless steel
D.aluminum alloy

A.lifted
B.reassembled
C.dismounted
D.disassembled

A.a low load on that cylinder
B.sluggish piston ring action
C.excessive piston crown cooling
D.excessive clearance of piston pin