单项选择题
A.removing heat and deposits
B.neutralizing acidic products
C.lubricating
D.improving combustion efficiency
单项选择题 The air distributor is conventionally driven by（） and distributes into each cylinder by（） certain sequence.
单项选择题 When an engine is starting and has got a sufficiently high speed, say about 20 rpm,（）
单项选择题 When starting an engine, the compressed air enters the cylinder when the corresponding piston is in position of its（）