该题选择B或 C，都正确。
如果被强调的成分是地点副词或时间副词，而不是介词短语，除了用that外，还可以用where，when引导。如果被强调的地点或时间是介词短语或从句，则不能用where, when，只用that引导。例如：　　

It was yesterday that (when) he bought an English book in town.

It was last week that (when) we worked in the countryside.

It is here that (where) he must be waiting for her.

last night 也是时间副词，不是介词短语。

It was in this room that we had a heated discussion.

It is at ten o’clock that the meeting will take place.

It was when he was criticized that he realized his mistake.

强调句貌似定语从句，但通过句子意思就可以判断出来。网友的句子若按照定语从句理解的话，语义不通。it 无所指。