You___hungry yet,for you had a big cake only twenty minutes ago.A.mustn't be B.can't beC.can't have been D.may not have been 最好有点分析的过程

答案：C
解释：
for （因为），有推断的意思
for you had a big cake only twenty minutes ago
因为你在二十分钟前刚吃过一个大蛋糕
所以根据推测，第一句用can't be，表示否定猜测
又因为有yet（仍然）用完成时态
翻译：
You can't be_hungry yet，for you had a big cake only twenty minutes ago
你现在不会仍然感到饿，因为你在二十分钟前刚吃过一个大蛋糕
综上，选C
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答案是C，这句话的意思是：你不可能现在就饿了，因为20分钟前，你刚吃了一个大蛋糕。yet是现在的意思，应该用现在时，排除CD。mustn't是不准的意思，排除A。答案是B，不可能。

I think is D.
B & C must not correct, so we only have A & D.
Before I think is A , but A is said "mustn't be"
when you put A. it means :
you must not be hungry yet.
But you don't know he / she is fat or thin,or he / she can eat so many things. so it would be D.
D is means:
you maybe are not hungry yet.
so the answer is D.

B.can't be-----------不可能
.mustn't --不准，不许可

B
对否定的猜测,只能用can't
你不可能饿了,你20分钟前才吃了个大蛋糕
希望能够帮到楼主