情态动词：She must have gone back to the vally.－She must have gone back to the vally.－No,she ___ the entrance to iy was no where to be found.A mustn't B couldn't C hadn't为什么

情态动词：She must have gone back to the vally.－She must have gone back to the vally.－No,she ___ the entrance to iy was no where to be found.A mustn't B couldn't C hadn't为什么
情态动词：She must have gone back to the vally.
－She must have gone back to the vally.
－No,she ___ the entrance to iy was no where to be found.
A mustn't B couldn't C hadn't
为什么

情态动词：She must have gone back to the vally.－She must have gone back to the vally.－No,she ___ the entrance to iy was no where to be found.A mustn't B couldn't C hadn't为什么
情态动词must在否定中不能用mustn’t.而且该句must对应的否定词只有couldn’t.所以选B

B 意思是她“不可能”，山谷的入口找不到
couldn't意思是不可能
一般否定句和疑问句用can，肯定句用must

must have +动词的过去分词是对过去动作的推测。must用于推测性用法时，只用于肯定句。疑问句和否定句用can 或could.这句话中，是否定的回答，所以用couldn't.

C hadn't