One of the main functions of wearing rings
题型：单项选择题
问题：
One of the main functions of wearing rings, as used in a centrifugal pump, is to （）.
A.maintain radial alignment between the pump impeller and casing
B.absorb all impeller shaft end thrust
C.prevent water leakage to the atmosphere
D.prevent wear of the pump casing and/or impeller
