Question

Fill in the blank with a suitable modal verb:
You ____ be mad to do this.

• A. shall
• B. must
• C. can
• D. could

Answer 1

Answer: (B)

must

Option B is the correct answer because 'must' is used to express compulsion, strong obligation, strong determination, probability or likelihood. it is stronger than should, in this case, the speaker thinks that he is probably mad so must is correct.

Option A is incorrect because 'shall' is used to give advice or make suggestions which is not correct here because the speaker is calling him mad.

Option C and D are incorrect because 'can' and 'could' are used to express ability or capability to do something.