B is 'wrong' simply because A is the idiomatic exclamation we use. Note that it ends in an exclamation point. If it were a real question (as B is), it would end in a question mark.
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AnonymousSo, this is my questionI think this is just one of those expressions you have to memorize.
- "Simon is moving to Hawaii"
- "Who ___ it!"
a. would have believed
b. could believe
The answer is obviously 'a', but why is 'b' wrong?