单项选择题 A positive displacement pump is that where the volume of the pump chamber is alternately （） to draw the liquid in and then （） to force the liquid out.

单项选择题 The pressure head of a water pump is given in the unit of （）.

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