单项选择题
（）, the engine must be stopped immediately.
A.Should knocking occur B.If knocking occur C.Should if knocking occurs D.If occur knocking
单项选择题 Grind the （）.
单项选择题 Which of the following can be obtained by means of an indicator card?（）
单项选择题 The lower section of a piston is called the （）.
