Mike is a careless student ,for this is the third time that he has made the same mistake ,______?答案为,isn't it为什么?为什么不是指代主句“mike is a careless student“而使用"isn't him"?或者是否可以理解为指代整个一段话?究竟这算是什么用法?为什么从来没有见过此类讲解?