—Why don't you go out to play, Rose?
—I'm afraid I can't. I have much homework _________.
A:do
B:does
C:doing
D:to do
—Yes, I'm really ______ in it.
改善地区大气质量的措施有( )
①强制性推广无氟电冰箱和空调 ②变城市居民的集中供暖为分散供暖
③大力推广汽车尾气净化技术 ④增加城市的绿地面积和公共空间
对一定质量的气体,若用n表示单位时间内与器壁单位面积碰撞的分子数,则( )