21. There is no doubt _______international cooperation is the key to dealing with cyber crime.
A. then B. that C. wheather D. this
22. As is known to all, China _______the group of developing countries.
A. belonging to B. was belonged to C. belongs to D.is belonged to
23.The visitors saw rows of houses the roofs _______are red.
A. where B. that C. what D. of which
