题目内容：

高中英语问题,高手帮下忙!
He asked his father _____.
A.where it happens B.where did it happen C.how it happened D.how did it happen. 答案是C,我想选B.请问B错在哪里呢?did后面不是跟动词原形吗?D答案有错在哪里呢?请给个详解!谢谢!