21. The bicycle-sharing system may help reduce air ________ (pollute) in big cities.
22. About 9,000 African elephants ________ (appear) in the 1990s. They’re in great danger.
23. Jane looks so _______(happiness) today because she has got an “A” in her math test.
24. _______(lucky), no one broke in because no one knew the password (密码).
25. The photos ________(take) many years ago and lost last week.
26. Our teacher, Miss Chen, ________(teach) English on the radio the day before yesterday.
27 A: Did you start to study food nutrition in 2017?
B: Yes, I ________ (study) it for several years and I am really interested in it.
28. We bought her a birthday present, ________ she liked it very much.
29. I won’t believe that the five-year-old boy can read five thousand words______ I have tested him myself.
30. The reason ________ he was late was that he got up late.