酸雨的出现，是因为空气中增加了大量的（　　）

A．二氧化碳

B．一氧化碳

C．二氧化硫

D．盐酸

It’s too bad that I failed in the math exam. If only I ______ hard at it!

A．worked

B．had worked

C．has worked

D．would work