Which of the following italicized parts is used as a predicative（表语)？A．I"m now downstair

Which of the following italicized parts is used as a predicative(表语)？

A．I"m now downstairs,waiting for you.

B．He runs downstairs to see his beloved girl friend.

C．China is a developing country.

D．The factory is developing quickly with the lead of an ambitious young woman.

请帮忙给出正确答案和分析，谢谢！