See the flags on top of the building?That was ________ we di

答案D
句意：“看到楼顶上的那些旗帜了吗?那是我们今天上午干的活.”that指“把旗竖起来”那件事.此处是what引导的表语从句,what在从句中作did的宾语.what还可引导主语从句和宾语从句.如：What I said is true．(what引导主语从句).I don’t believe what he said．(what引导宾语从句).that也可引导名词性从句,它与what的区别在于that在从句中不充当任何成分,只起连接作用,引导宾语从句还可省略,而what则充当成分,且不能省略.