单项选择题
Black smoke coming from a boiler can be caused by （）.
A.low water level B.low oil pressure C.excessively high fuel pressure D.an improper air/fuel ratio
单项选择题 Which of the following pumps belongs to centrifugal pump?（）
单项选择题 The advantage of a centrifugal pump as compared with a reciprocating pump is that （）.
单项选择题 A pump is said to have "negative suction head" when the pump is located （）.
