you must be fifty,___?

题目

you must be fifty,___?
A.mustn't you B.needn't you C.aren't you?D.needn't you
为什么?不太懂

答案

选项是C. 因为这里的must表推测,意思是：“想必你50岁了,是吗?”must不是表“你一定是/必须是50岁”.