The engineer must be at home,for the light is on,___?

题目

The engineer must be at home,for the light is on,___?
A.mustn't he
B.isn't he
why we choose B?

答案

must表示判断时,反意疑问句仍然按照原来那个句子来问,isn't he
must表示必须时,如 we must go home now,mustn't we?