单项选择题
Which of the following statements concerning fire-tube boilers is correct?（）
A.Combustion gases flow through the tubes B.Flames impinge on the tubes C.Combustion occurs in the tubes D.Water flows through the tubes
单项选择题 A diesel engine cooling water system with a pH factor of 30 indicates a condition of （）.
单项选择题 Fuel injectors used in heavy fuel oil systems are usually provided with cooling to reduce（）.
单项选择题 The rate of fuel injection in a diesel engine cylinder depends primarily on （）.
