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．Of all the subjects, I like history the best because it gives us useful knowledge of things in the past.
A．the; a B．/; a C．a; the D．a; /
22．—What were you up to when your parents came in?
—I for a while and some reading.
A．was playing; was going to do B．played; did
C．had played; was going to do D．had played; did
