21．As Susan stared at ______ piles of plastic in her recycling bin, ______ strong sense of responsibility made her stick to her recycling work．
A．不填；不填 B．不填; a
C．the ; a D．the; the
22．Tom's mother kept telling him that he should work harder, but ______didn't help．
A．he B．which C．it D．one
23．Put yourself in situations where you're forced to communicate in English, ______ you'll see more progress over time．
A．until B．or C．but D．and
24．The United Nations are evaluating the damage caused in Haiti earthquake, while in Chile the government itself ______ the lead role．
A．is taking B．was taking C．took D．had taken
25．What's your opinion of Mr．Li's request that we ______ spend half an hour reading English aloud every morning?
A．would B．should C．must D．could