she must (go out)___because nobody answered the doorbell空里为什么填have gone out?
问题描述：
she must (go out)___because nobody answered the doorbell空里为什么填have gone out?
答
因为句子意思是她肯定是出去了因为按门铃没人应.
这时她已经出去了,用完成时那为什么不用过去完成时呢？Q_Q因为前面有个must后面不能用过去式啦。情态动词后面一定要跟原形