`I`.
13. hasn't done (yet: HTHT)
14. visited (last year: QKĐ)
15. plays (usually: HTĐ)
16. to take/taking (like + to-V/V_ing: thích)
`IV`.
17. Yes, they do (câu đầu)
18. For ten weeks (Each expedition lasts for ten weeks and takes 150 volunteers)
19. They help scientists to do environmental research (Others work in national parks or help scientists to do environmental research)
20. English (To become a volunteer, have to be between 17 and 25; you have to speak some English [...]