单项选择题
A.prevent the gear box from leaking
B.prevent the oil from becoming inflammable
C.make sure it has not become contaminated
D.make sure the motor bearings are lubricated
单项选择题 A spur gear pump should be operated with the discharge valves （）.
单项选择题 The capacity of a centrifugal pump can be increased by the installation of a larger diameter impeller Another means of increasing the pump capacity is to （）.
单项选择题 Which of the following forms of energy is demonstrated through the process of combustion?（）