单项选择题
Which features of a centrifugal pump reduce the need for renewing worn impellers and pump casings?（）
A.Close radial clearance between impeller hub and casing
B.Low rotational speed of impeller
C.Removable end plate
D.Replaceable impeller and casing wearing rings
单项选择题 When securing a flash-type evaporator for an extended period of time, you should （）.
单项选择题 A distinguishing feature of an educator, when compared to other pumps, is the （）.
单项选择题 The heat required to change a substance from a liquid to a gas without experiencing a temperature change is defined as the latent heat of （）.