【一道定语从句题Thehousewhichheboughtin1999,____hesoldtwoyearslater,isagainonthemarket.A.andthatB.whichC.andwhichD.when1,有如下规则“有两个定语从句,其中一个关系代词已用which,另外一个宜用that”,为】

　　一道定语从句题

　　Thehousewhichheboughtin1999,____hesoldtwoyearslater,isagainonthemarket.A.andthatB.whichC.andwhichD.when

　　1,有如下规则“有两个定语从句,其中一个关系代词已用which,另外一个宜用that”,为什么这里用两个which

　　2.为什么which前要加and