___of the land in that district___covered with tree and grass
15. --- Who are the teenagers gathering at the school gate? --- Well, if you _____ know, they are all my visiting students.
16. Nothing taught by others can have the same effect on you as _____ learned by yourself.
17. In China, _______ bicycle is ______ popular means of transportation.
18. --- What do you think of the furniture on exhibition? --- Well, great! But I don’t think much of _______ you bought.
19. The best job is _______ which uses your skills in doing something together with your interest in the subject.