Which of the following contains a subject clause？A．It is a fact that he has done his best.

Which of the following contains a subject clause？

A．It is a fact that he has done his best.

B．It is the best film that I have seen.

C．It is said that she has passed the exam.

D．It is Jack that will look after the old man.

请帮忙给出正确答案和分析，谢谢！