从A、B、C、D四个选项中，选出可以填入空白处的最佳选项，并在答题卡上将该项涂黑。
21．In many places in China, _____ bicycle is still _____ popular means of transportation．
A．a； the B．/; a C．the； a D．the; the
22．The road is covered with snow．I can't understand they insist on going by motor-bike．
A．when B．why C．whether D．how
23．Your floor is dirty．It needs _____．
A．cleaning B．to clean C．clean D．cleaned
