单项选择题
（）does it take to change the main engine from ahead to astern?
A.How many times B.How much water C.How long D.How length
单项选择题 （）, we continued lifting out the pistons.
单项选择题 The exhaust valve is opened at about the same time （） the air inlet ports are opened.
单项选择题 When entering a foam injected space （） breathing apparatus.
