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At the same time there is a little water on the planet. In many lakes and rivers the water is very dirty. Sometimes people can`t swim in seas, rivers and lakes because water is dirty. In many places water is not able to drink and it is dangerous to use it when you cook. Even fish die in such water.

Some people`s activities do a lot of harm to the forests. People cut down trees to build farms, homes and roads. Many animals and plants lose their homes. Many of them die. This is bad for the air of the Earth too. Modern plants and factories send a lot of smoke in the atmosphere. This is very bad, because nowadays it`s difficult to breathe in big cities.

1. I must do it today: it is the last day.
2. May I open the window? – No, you must not.
3. We must not take the teacher’s book.
4. We don't need to come to school in June. There are no lessons.
5. Will he come to see us? – I don’t know. He may or he may not.
6. Shall I phone him today? - No, you must not, he is busy, but you can phone him tomorrow. 7. Must (May) I cut it without a knife? – No, you must not, it’s dangerous.
8.You need not  have bought the vegetables: we have everything for dinner.
9. Why can't you understand it? It is so easy