The mother didn't know who the glass because it happened when she was out.
A. break
B. has broken
C. had broken
D. breaks
网友回答
A. break
B. has broken
(答案→)C. had broken
D. breaks
解析：考查时态。句意：妈妈不知道是谁打破了杯子，因为这是在她外出的时候发生的。杯子坏了是在妈妈知道之前，而本句的妈妈知道都使用了过去时，故打破杯子使用过去完成时。故C正确。