与异常的宿主反应有关的牙周炎是
A．侵袭性牙周炎B．顽固性牙周炎C．慢性牙周炎D．复合性牙周炎E．成人性牙周炎
参考答案：A
---How about going shopping this afternoon?
----This afternoon? I don’t think I will be free then.Let’s make ___ some other time.
A．that
B．one
C．it
D．this
试举例说明市场失灵的原因和对策。
