单项选择题 In a four-stroke engines the camshaft rotates at （）.

单项选择题 In a diesel engine, fuel must be broken up into a spray of fine droplets to （）.

单项选择题 In a complete oil/water separator, more oil will separate out onto the underside of these plates and travel outwards until it is free to rise into the oil collecting space Whats the meaning of "it" in the sentence above?（）