21
[答案]:C
[解析]:参见教材P200
22
[答案]:D
[解析]:参见教材P190
23
[答案]:B
[解析]:参见教材P199
24
[答案]:A
[解析]:参见教材P190
25
[答案]:C
[解析]:参见教材P192
26
[答案]:B
[解析]:参见教材P201
27
[答案]:C
[解析]:参见教材P202
28
[答案]:C
[解析]:参见教材P202