用括号中所给的动词的适当形式填空。 1. You must _____ (look) after your pet dog. Don't let it run here and there. 2. —What's behind the door? —There _____ (be) two balls. 3. —Why not _____ (sing) an English song together? — Good idea. 4. There _____ (be) some money in that purse. 5. We are planning _____ (go) for a picnic this Sunday. 6. It often _____ (rain) here in summer. 7. You must remember _____ (close) the window when school is over. 8. The students are busy _____ (do) their homework. 9. _____ (skate) is my favorite sport. 10. There _____ (be)a pen and some books on the desk.