阅读短文，从A、B、C、D四个选项中选出可以填入空白处的正确答案。 When people talk about air pollution, they are usually thinking about outdoor air 1 . But do you know that there is also air pollution inside homes, offices, hotels and other buildings? The air in your home can be 2 to 100 times more polluted than the air outdoors! In fact, some American doctors say that 50% of the illnesses have 2 to do with the polluted indoor air. A lot of pollution comes from indoor activities 3 smoking and cooking. As most people 4 about 80%-90