What is the difference between ‘must’ and ‘have to’?

A. Must is used for personal preference, while have to is used for necessity.

B. Must is used for past obligations, while have to is used for future obligations.

C. The main difference is that ‘must’ expresses a strong obligation based on the speaker’s opinion or authority, while ‘have to’ expresses an external obligation imposed by external factors or rules.

D. Must is informal, while have to is formal.