单项选择题
A.is coming upward；cylinder ring belt
B.is going downward；piston ring belt
C.is coming upward；piston ring belt
D.is going downward；cylinder ring belt
单项选择题 （）diesel engines generally have two systems of lubrication:（）a total loss system feeding and a circulating system lubricating the running gear and cooling the pistons.
单项选择题 The following options are all the functions of lubricating oil except（）
单项选择题 The air distributor is conventionally driven by（） and distributes into each cylinder by（） certain sequence.