Which of the following italicized parts is used as a subject？A．Mr. Smith said he found it

Which of the following italicized parts is used as a subject？

A．Mr. Smith said he found it hard to get along well with her.

B．The teacher asked us to collect more information about the subject.

C．It was pretty hard for him to bring up the child on his own.

D．The principal told me that my job was to teach them English.

请帮忙给出正确答案和分析，谢谢！