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
A.(1), (2) and (3) B.(1), (2) and (4) C.(2), (3) and (4) D.None of the above
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.
单项选择题 Sediment collects in a tubular purifier （）.
