是aren't you?
因为这是特殊的反义疑问句,主句表推测,反问按主句的推测时态来写.
对现在推测则用现在时反问,如You must be on the bus,aren't you.
（我猜想）你现在一定在车上,是么?
对过去推测则用过去时反问,如You must have been on the bus,weren't you.
（我猜想）你当时一定在车上,是么?
会了么?