Поясните, почему необходимо сохранять разнообразие биогеоценозов в биосфере.

1. She …..(cooked) yesterday.
2. She ……(was cooking) at 3 o’clock yesterday.
3. She….. (was cooking) from 2 till 4 yesterday.
4. I….. (was not reading) when you ……(came) in.
5. What…(was Peter doing) when I….. (phoned) him?
6. When my father ……(came) home I….. (was having) dinner.
7. When Alice ….( returned) he …..(was listening) to the radio.
8. When her aunt….. (was airing) the room she …..(caught) a cold.
9. The young people……. (were dancing) when I……. ( came) to the party.
10. When the teacher….. (entered) the classroom the pupils……. (were sitting) at their desks.