— Why do you keep the windows open on such a cold day?

— There are so many students here, so we want to keep the air nice and _____. [ ]

A. cold

B. dry

C. cool

D. clean

市场失灵具有必然性，常见的市场失灵包括（）

A、外部性

B、内在性

C、缺乏必要的强制

D、寻租现象