存在正外部效应的情况下，这种物品或服务的私人边际效益 小于其社会边际效益。
参考答案：对
Li Lei hasn't come yet. I don't know what ________ to him. [ ]
A. is happened
B. has been taken place
C. has taken place
D. has happened
A．I= ，从上向下
B．I= ，从上向下
C．I= ，从下向上
D．I= ，从下向上
