A multistage centrifugal pump can be BEST described as having （）.

A. two or more impellers housed together in one casing and mounted on a single shaft
B. a single stepped impeller mounted in a progressively staged casing
C. a large radial clearance between the impeller and casing to prevent overheating due to
D. an increase in the discharge velocity of the liquid with a corresponding decrease in pressure through the stage