Must or Could

[Kaito-Hacker]

Haven't ou finished writing your letter yet? You _____ it for two hours!
A. must be writing
B. could have been writing
C. must have been writing

I think the answer is B because the sentence sounds like uncertain.
Please let me know if i'm wrong. An explanation will be appreciated!

[5jj]

Haven't you finished writing your letter yet? You _____ it for two hours!
A. must be writing
B. could have been writing
C. must have been writing

I think the answer is B because the sentence sounds like uncertain.

The answer is C. The speaker is saying that s/he is pretty certain about the time period. This fits with the previous question.

'Could' or 'might' are unlikely when there are only two peole involved in the dialogue - at least one of the speakers knows. They are more possible when speaking about a third person:

Fred when to his room to write some letters after breakfast. I suppose he could have been writing for six hours.

[Tdol]

I agree. Where did you see this test? Was it on this site? If so, I will change it.