Which of the following sentences has a formal subject（形式主语)？A．It was 5 o"clock when he

Which of the following sentences has a formal subject(形式主语)？

A．It was 5 o"clock when he came here.

B．It is no use trying it again.

C．It was yesterday that I met her.

D．I find it easy to get on with her.

请帮忙给出正确答案和分析，谢谢！