问题详情

What is the problem with hip or knee replacement in the U.S.？
A．A lot of patients need a second operation.
B．Doctors are not well trained to ensure successful operation.
C．Demands for hip replacement exceed the number of surgeons.
D．Replacement operation is becoming too expensive in U.S. hospitals.请帮忙给出正确答案和分析，谢谢！