单项选择题
The most important factor that causes the wear of the engine parts is （）.
A.excessive heat B.friction C.oxidation D.high pressure
单项选择题 In the closed cooling system, the water （）.
单项选择题 If the temperature drops low enough, oil will not flow, （） just tell us the temperature.
单项选择题 The （） can tell us how much heat we can get from the burning of an oil.
