21. Chatting online can be _____fun, but kids must be sure they stay safe on _____web.
A. 不填; a B. a; the C. 不填; the D. a; a
22. Life is like a long race _______ we compete with others to go beyond ourselves．
A. which B. what C. that D. where
23. It’s a common scene in the filmTangshan Earthquakethat families cry helplessly over _____ is left of their homes.
A. what B. which C. there D. it