问题
选择题
---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
答案
答案：D
在寒冷的天气把窗户打开是为了使空气清新和干净，clean表示此意，故本题选D。