Choose the correct option

16. Sometimes it's better not to buy something new, and buy it ____ instead.

A. used                        B. usable                     C. use                          D. useful

17. He asked the children ____ the river.

A. don't pollute           B. if they don't pollute            C. not to pollute          D. not polluting

18. They asked me how many students ____ in the school.

A. there were              B. were there               C . there are                D. are there

19. He told me that ____.

A. he would participate in the conservation campaign yesterday

B. he had participated in the conservation campaign the previous day

C. he participated in the conservation campaign the previous day

D. he has participated in the conservation campaign the previous day

20. He asked, “Why did she write the article on conservation?”

      He asked why  ____.

A. she had written the article on conservation           B. she has written the article on conservation

C. she wrote the article on conservation                    D. did she write the article on conservation