Is this ----looking for?This is ------looking for.

Is this that you were looking for?This is what you are looking for.
首先第一个是个问句.还原成陈述句：
this is something that you were looking for.可以看出是对主句的宾语提问.同时主句的宾语是从句中作宾语.对something 提问就成了：is this that you were looking for?
第二句中是陈述句.主句中少宾语.必须要用what.同时作主句和定语从句中的宾语.
再问： 好吧……刚跟老师对了答案，其实两个空都选B。两个句子都缺宾语。
再答： 好吧，仔细想了想，是我错了， 对不住你了。