单项选择题
Here some oil in the oily water separator, as a result of its lower （）, will separate and rise into the oil collection space.
A.temperature B.viscosity C.density D.velocity
单项选择题 Heat energy is a source of power, but （） will cause damage to the engine.
单项选择题 Head tank should be used to remove （） from cooling water.
单项选择题 He said that he had turned the main engine one complete revolution with the （）.
