单项选择题
（）sometimes called filter, is a device designed to prevent the passage of unwanted solids into the system.
A.Steam trap B.Check valve C.Separator D.Strainer
单项选择题 Hydraulic pumps most commonly used in steering systems are of the （）.
单项选择题 When an auxiliary boiler is panting and emitting black smoke, you should （）.
单项选择题 Close the （）.
