Here are the best answers for the sentences:

1. **A**: are knocking (Listen! Someone **is knocking** at the door.)
2. **B**: I see (Oh dear! **I see** that I forgot my calculator.)
3. **C**: knock (I’m excited **to knock** our first day at school.)
4. **A**: with (Let’s **wear** our school uniform and go to school.)
5. **B**: wear on (Get up, Tom. We **have on** our school uniform today.)
6. **B**: going back (We **are going back** to school today.)
7. **A**: at (I meet my friend **at** my way to school.)
8. **B**: help (Do you **help** your things with your classmates?)
9. **B**: by (My school is **surrounded** by mountains and green fields.)
10. **D**: all are correct (How do you get to school every day? **All are correct**.)
11. **A**: have (I do my homework with my friends. **I have**.)
12. **B**: international (Students live and study in a **international** school. They only go home at weekends.)
13. **A**: When (What time does Mai usually cook dinner? **When** is the time?)
14. **B**: do judo (They are healthy because they **do judo** every day.)
15. **A**: play music (study vocabulary, **play music**.)
16. **A**: Yes, I do (Do you keep quiet when your teacher is talking? **Yes, I do**.)
17. **A**: Yes, I am (Is she a nurse? **Yes, I am** & a & b are correct.)
18. **D**: d & c are correct (Hoang lives in a small house in the center of the city. **d & c are correct**.)

Let me know if you need any more help!