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？不会再问。