It ___ be Thai or Indonesian, I think! (must / can't /mustn't)

6 anni fa

Hello!
There's a test question that bothers me. There's a gap to be filled.

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!

P.S.: It is known that 'must' can imply strong probability which means that the speaker thinks that it is probably Thai or Indonesian food. 'can't' vice versa shows that it's highly unlikely that the food is Thai or Indonesian that make both variants grammatically correct

Is taken from: English File Intermediate (Oxford)