IV.
21. a -> an
22. polluted -> pollution
23. noisy -> noisier
24. Lucky -> Luckily
25. learning -> to learn
V.
26. visit her friend because she doesn't have time.
27. how much I thought it would cost.
28. gone aboard before.
29. been used for three months.
30. is considered to be one of the best.

21.a->an(hour không phát âm chữ h)

22.environmental->environment

23.noisy->noisier(sau có than phải dùng so sánh hơn nhất)

24.Lucky->luckily(diễn tả trạng thái may mắn)

25.learning->to learn

26.She won't visit her friends because she has no time.

27.He asked me how much I thought it would cost.

28.He hasn't gone abroad ever before.

29.That computer has been used by them for 3 months.

30.His speech is considered one of the best.

Học tốt!