17/ There are a lot of people here,______?    a. are they                             b. are there                  c. aren’t there              d. aren’t they 18/ They had to go home,______?    a. hadn’t they                       b. didn’t they              c. had they                  d. did they 19/ We should avoid_____our environment.    a. to pollute                          b. polluting                  c. polluted                   d. pollute 20/ He apologized for_______me______so long.    a. keeping/ waiting    b. keep/ wait              c. keeping/ to wait       d. to keep/ waiting 21/ It ‘s too hot now. Why don’t we go_____?    a. to swim                             b. swimming                c. swim                        d. to have swim