She ________ a model before she _______ a superstar.

A．have been, became

B．had been, become

C．had been, became

D．has been, become

造成电动机绝缘下降的原因是（）。

A.电动机绕组受潮

B.引出线及接线盒内绝缘不良

C.电动机绕组长期过热老化

D.上述三种说法都对