If you must know, his name is Peter.中must是什么意思

问题描述:

If you must know, his name is Peter.中must是什么意思
— Who is the boy standing over there?
— Well, if you _______ know, his name is Peter.
A.must B.may C.canD.shall
答案是A,为什么那,谢谢

MUST这里当一定的意思,其他三个不符合啊,MAY表示可能,CAN用这里不对,SHALL表将来,MUST较合适,表示你一定要知道的话,他的名字叫PETER