某文章的“主角”是一氧化碳,该文章的标题可能是( )
A:PM2.5与雾霾天气
B:新装修屋内的空气杀手
C:酸雨的罪魁祸首
D:室内燃气热水器的安全防范
While we _______ football in the classroom, the teacher came in.
Having retired from business, he now ____(忙于) himself with the welfare of the disabled.