Which of the following statements is/are true regarding hydraulic pumps in general? （）
A. Variable volumes can be obtained with gear pumps only by variation of the pump drive speed
B. A radial piston pump houses sliding pistons in a stationary cylinder block through which passes a rotating pintle or ported shaft
C. The amount of fluid displaced per revolution of an axial piston rotary pump is dependent on the angle formed between the cylinder block and valve plate
D. All of the above
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