一、将下列句子改写为被动语态。 1. A car knocked him down yesterday. 2. Two doctors and ten nurses make up the medical team. 3. When I got there, they were cutting up a fallen tree. 4. We'll put on the play next Sunday. 5. Workers are building a new teaching building in our school. 6. They had completed the railway by the end of last year. 7. We should protect the earth. 8. You need to paint the wall.