!! Нужно поставить в правильное время!!
1. When gasoline is burnt to power cars and trucks, it (produce) carbon monoxide.
2. Carbon monoxide is harmful when it (concentrate) in great amounts.
3. If the air in a city (get) very polluted, it can make people’s eyes burn.
4. If smog is frequently formed in a city, it (cause) difficulty in breathing, headaches and even
lung cancer.
5. Some cities that suffer from extreme smog, issue air pollution warnings. If you (live) in Hong
Kong, you would often hear official warnings not to go outside or engage in strenuous physical
activity (such as running or swimming).
6. Unfortunately, we only recently discovered how air pollution contributes to global warming. If
we (know) about it earlier, we might have taken stricter measures in time.