单项选择题
The pressure head of a water pump is given in the unit of （）.
A.kg B.MPa C.mH20 D.None of the above
单项选择题 Which of the following are components of a mechanical seal? （） (1) stationary ring (2) rotational ring (3) packing (4)spring
单项选择题 The gear, screw, and vane pumps are all classified as （）.
单项选择题 The （） discharge a nearly constant amount of liquid regardless of pressure.
