以下不属于退出壁垒的是（）。
A、沉没资产壁垒
B、员工解雇壁垒
C、契约壁垒
D、规模经济壁垒
参考答案：D
There is little food left in the ice box. We want some tomorrow. [ ]
A. buying
B. to buy
C. have bought
D. be bought
下列四幅图中，能正确反映空气流动的是
A．① ②
B．③ ④
C．② ③
D．① ④
