80年全国研考(A)有这样一道试题：
Comrade Li ____ be in Beijing because I saw him in town only a few minutes ago.
A.mustn't B.can't C.may not D.isn't able to
标准答案要求选 B。但不知选 A 是否正确？
我国出版的许多英语语法书和英语试题都特别强调：must 用于表示“推测”（“一定”“必定”）时，其否定形式只能是 can't，而绝不是 must not（mustn't）。例如，She must be asleep. = She can't be awake.（《实用英语语法》）。情况是否真的如此呢？让我们先看两个反例：
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I can hear the Browns' phone bell ringing but no one is answering it; they mustn't be at home. (Eckersley) 我能听到布朗家的电话铃在响,但是没有人接电话;他们一定不在家。
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His absence must not have been noticed. (CGEL) 他的缺席一定没有人注意到。
再来看看 R. Quirk 等人在 CGEL 中的论述：
"The auxiliary negation of must in the logical necessity sense is usually achieved through can't: They must be telling lies. [‘It is certain that they are telling lies.’]
~ They can't be telling lies. [ 'It's not possible that they are telling lies.' ]
But must not and mustn't occur in this sense occasionally in AmE and, still more occasionally, in BrE. This use (instead of can't) seems to be gaining favour."
(“助动词 must 在作逻辑上的‘必然’解时,其助动词否定是通常通过 can't 来实现的。
但是,在美国英语中有时也会见到 must not, mustn't 这种否定形式,在英国英语中也有人这么用,只是用得比较少一些而已。这种用法(代替 can't)看来越来越受到人们的喜爱。”
我想，上面的反例再加上R.Quirk等语言学家的论述已经清楚全面地回答了你的问题。是不是？
在编写本书的第四次增订版时,我们又发现了新的例证:
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My cellular phone must not have been receiving, Aringarosa thought as the Fiat approached the exit for Rome's Ciampino Charter Airport. (Dan Brown, The Da Vinci Code) 我的手机肯定一直没信号，阿林加洛沙坐在菲亚特轿车中琢磨着，此时他们正直奔罗马的洽米皮诺机场。（《达·芬奇密码》）
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Mr. Green mustn't have received my letter, otherwise he would have replied before now. (Eckersley) 格林先生一定没有收到我的信，要不然他在这之前早就回信了。