几个语法题，谢谢

31. Statistics indicate that approximate every 22 years--within a range of 3 to 4 years--a major

drought occurs in the United States.                  A

28. Paleoanthropologists examine fossil remains of extinct primates, while physical

anthropologists concern with ethology study the behavior of primates in their natural

settings.                                                     B

19. Art Deed, a style of design popular in the 1920's and 1930's, was used primarily in furniture, jewel, textiles, and interior decoration.          C                       

为什么呢？