问题
When a hydraulic vane pump is working, which of the following is in contact with the vanes? (1) the rotor; (2) the internal surface of the housing; (3) the external surface of the housing.
A.(1) only
B.(2) only
C.(1) and (2)
D.(1), (2) and (3)
请帮忙给出正确答案和分析,谢谢!