选词填空。 1. Have you done your homework _____? (for, just, yet) 2. I have _____ been to Chengdu. (ever, never, yet) 3. He has _____ washed the dishes. (already, ever, for) 4. He has worked there _____ he left school. (already, just, since) 5. Have you cleaned your room _____? (for, just, yet) 6. She has known Sam _____ nine years. (for, never, since) 7. They have _____ arrived. (ever, just, yet) 8. Have you _____ ridden a horse? (ever, for, yet) 9. She has known Fred _____ they were at primary school. (already, just, since) 10. We haven't completed the project _____. (already, for, yet)