单项选择题
Which of the following may cause that an engine doesnt turn on starting air?（）
A.too low starting air pressure B.main starting valve closed C.neither A nor B D.A and B
单项选择题 Poor combustion in a diesel engine can be caused by （）.
单项选择题 A built-up exhaust valve is one in which （）.
单项选择题 If scavenge box should be hot, （）would occur.
