单项选择题
According to the principle of four stroke diesel engine, what is the number of its efficient work stroke per revolution?（）
A.1/2 B.1 C.2 D.3
单项选择题 What type of electric motor is most commonly used on board ship?（）
单项选择题 Which of the following statements is not true concerning symptom of a fire in the scavenge air box?（）
单项选择题 Which of following functions is not one of the functions of air conditioner?（）
