一道英语选择题,

题目

一道英语选择题,
--Someone is knocking at the door.
--Yes,it _____be Tim .I saw him outside from the window just now.
A.could B.must C.may
请给出答案并详细说明理由,thanks very much!

答案

根据问题的回答Yes,it _____be Tim .后的I saw him outside from the window just now这句
可以看出,回答者看到了Tim,所以应该是一种肯定的语气,所以选B must