21. High school is_____ time of discovery, learning and hard work.

A. the                            B. an                          C. a                            D. /

22. Wuhan-Guangzhou high-speed railway line with the world's fastest train – traveling ______ 350 kilometers per hour – began operations _____December 26. The 10-hour journey is now down to under three hours.

A. at…in                      B. at…on                    C. in …in                    D. in…during

23. Was ___in the supermarket ______I came across his mother the other day.

A. this…that          B. this…where        C. it…that           D. it…where

24. Antarctic _____ we know little is covered with thick ice all the year round.

A. which             B. where                     C. that               D. about which