下列句子中“之”字用法与其它三项不同的是 [ ]

A、学而时习之

B、昂首观之

C、见渺小之物必细察其纹理

D、诲女知之乎

All the people in the nation can remember exactly how many years ago _______ the civil war broke out.

A．was it that

B．it was when

C．it was that

D．was it when