“No one can be indoors”还是“No one must be indoors”？
mustn't只有不许的意思，没有表僦推测的意思－－－肯定不这是大家都知道的．但问题是：
A No one can be indoors.
B No one must be indoors.
A句不用说是正确的，Ｂ句呢？
找不到不能用Ｂ句的权威论证
请有这方面定论的论著说明并注明出处，谢谢
IsaacZ找到了这道题：
He had been knocking at the door for a long time, but no one came to open it. He said to himself, “No one _______ in doors.”
A. must be B. can be C. may be D. should be
以下是引用网上的讨论：
楚天阔：
must 表示猜测只用于肯定句中
can 表示猜测用于否定句或疑问句中
changcheng ：
Actually, depending on the context, they could mean a number of things including what Rovi has suggested. To know what the sentences really mean, therefore, you would need to know the context.
(It is "indoors", not "in doors", by the way.)
Some other meanings:
Can could also imply a physical possibility based on some known facts:
"I saw everyone leave a few minutes ago. No one can be indoors now."
Meaning, I saw every person leaving, so it is physically impossible that anyone is still inside.
Must could also mean a conclusion of one possibility based on some known facts:
"The phone has been ringing unanswered for a long time. No one must be indoors."
Meaning, because the phone has been ringing for a long time, we assume that no one is available indoors to answer it.
joham50 ：
No one must be indoors.最常见的意思是“任何人都不可以呆在室内。”但读过美国《西蒙高级（或中级）英语语法》的人都会记得mustn't可以用于表示推测，等于can't：
FUNDAMENTALS OF ENGLISH GRAMMAR, 5-10, by Betty Schrampfer Azer:
1. Eric ate everything on his plate except the pickle. He must not like pickle.
2. --I've knocked on the door and have rung the doorbell several times. Nobody has answered the door.
--Eric must not be at home. （美国英语中不兴mustn't的拼法）
这种用法在英美国出版的英语词典中好像都没看到，因此考试中一般都作错误答案。
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