这个句子是克漏字的一部分
By the time they started to feel better, it was already the sixth day and
they ___ the next day.
(A)must leave
(B)had to leave
(C)was to leave
(D)want to leave
答案：B
想问的是
为什么答案不能选A

must+原形不能用于过去had to或ought to可以倒退，must不可

谢谢回答