Have to vs. must

[John]

Test No. incompl/inter-73 "Modal Medley", question 9

Now there's no argument here. You ......... to finish on time because you have no choice.

(a) must
(b) can
(c) have

Test No. incompl/inter-73 "Modal Medley", answer 9

Now there's no argument here. You have to finish on time because you have no choice.

Correct answer: (c) have

Your answer was: incorrect
Now there's no argument here. You must to finish on time because you have no choice.
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Why "must" doesn't fit

[Alan]

Must is NOT followed by a preposition.
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