who is that?it must be Mary.为什么不能用she?

who is that?it must be Mary.为什么不能用she? Who is that?( )Mary A it's B she's C that's she knew that it must be the baker 为什么不是she knew that it mus That -------- be Mary.She is at home. 为什么不能用must to be? Mary is so devoted to whatever she is doing that it won't be -Who is singing in the next room.-_ must be Marie A.It B.She who is singing?it must be susan.为什么是用it 英文打电话问“你是谁?”能不能用“Is that Mary?“,为什么?麻烦说的详细一点 为什么不能用it is sure that,而可以it is certain that who is the girl （that）is crying.为什么不能用which? She is not the cheerfuf girl that she was.为什么只能用that引导,而不能用w