Which of the following italicized parts is a subject clause（主语从句)？（2011)A．It is sheer

Which of the following italicized parts is a subject clause(主语从句)？(2011)

A．It is sheer luck that the miners are still alive after ten days.

B．He has to face the fact that there will be no pay rise this year.

C．She said that she had seen the man earlier that morning.

D．We are quite certain that we will get there in time.

请帮忙给出正确答案和分析，谢谢！