31. Air pollution and noise pollution are harmful ______ people’s health.

A. in                    B. on                 C. to

32. –What did Mr. Wang say just now?

   –He said that the earth ­­­_____ around the sun.

A. went                 B. goes               C. is going

33. –How many classes do you have on Friday afternoon?

   – ______.

A. None                B. Nothing             C. Nobody