Nothing is impossible if you put your heart into it (改为反义疑问句

Nothing is impossible if you put your heart into it (改为反义疑问句)Nothing is impossible if you put your heart into it ____ ____?
记得老师讲过，impossible这样的有否定前缀的词在句中还算肯定，所以不应该是isn't it 答案上却说is it？为什么？