Test question from English File Intermediate (Oxford)
There's a test question that bothers me.
A What kind of food is this?
B I'm not sure. It ___ be Thai or Indonesian, I think!
A must B can't C mustn't
I know "mustn't" is wrong as it expresses prohibition. So, I really need to choose between A must B can't. Could you explain why option A is way more better than option B? There must be grammar rules that explain this case.
Thank you very much!