请认真阅读下面各题，从题中所给的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. Below the topsoil of the earth is the subsoil, _____ layer that contains stones mixed with a small amount of _____ organic matter.
A. a; the B. the; the C. 不填; 不填 D. a; 不填
22. —Mike, have you ever heard of the latest water heater using solar energy?
—Yes, of course, but I haven’t decided whether to buy _____.
A. it B. one C. this D. that
