Необходимо исправить ошибки в предложениях1. What are you doing when the

Необходимо поправить ошибки в предложениях
1. What are you doing when the police arrived?
2. Mary can't answer the questions. She hadn't read the text yet.
3. It is often snow in December.
4. The lesson started since twenty minutes ago.
5. These pupils have finished Unit Fifteen last week.
6. They have been written the test for an hour.
7. John is doing exercises since 8 o'clock