----She must have gone back to the valley. ----No she . The entrance to it was nowhere to

问题补充：

----She must have gone back to the valley.

----No, she . The entrance to it was nowhere to be found.

A mustn’t B needn’t C couldn’t D hadn’t

答案：

解析：must have gone表示对过去事情的肯定推测，答话了否定的，所以这里用couldn’t表示不可能。