30. Ayou want to have repaired作定语修饰the TV set，其中you省略了定语从句的引导词that/which，引导词在从句中作have的宾语。因此此处实际上是词组have sth done结构。这儿的sth就是省略的引导词that/which。31. a fourth one表示“又一个”（在三个的基础上）。只有表示“特指第四个时”才用the fourth。如果泛说“又一个”要用a+序数词+名词。如：He asked a studenta question, but the student couldn't answer. so he asked a sceond one....
