一道初二的题：must be 与 may be

一道初二的题：must be 与 may be
I'm afraid I______be going.It's too late.
A.must B.can C.need.D.may
在本题中,must be 与may be 的区别到底在哪呢?
另有类似一题：
Who is that boy in a red coat?
It ____ Tom.He likes red coat.
A.must be B.can be C.may be