从A、B、C、D四个选项中，选出最佳选项，并在答题卡上将该项涂黑。
21．— Have you seen ____ pencil? I left it here this morning．
—Is it _____black one? I think I saw it somewhere．
A． a; the B． the; the C． the; a D． a; a
22． As many as 100 species of fish, some ____ to these waters, may have been affected by the pollution．
A． relevant B． immune C． unique D． convenient
23．—Did Hans take part in the contest?
—No, at the last moment his courage _____ him and he ran away．
A． ignored B． declined C． failed D． lost
24． All her hard work ______ in the end, and she finally passed the exam．
A． paid back B． paid off C． brought out D． brought off
