---Why do you keep the windows open on such a cold day?-There are so many students here

问题补充：

---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：

A. cold

B. dry

C. cool

(答案→)D. clean

解析：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。