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II/
1. If we go one littering, the environment will become seriously polluted.
2. Tet is the most important celebration for Vietnamese people.
3. I'm pleased that you are working hard.
4. She likes reading books which tell about different peoples and their cultures.

III/
1. Tet is a festival which occurs in late January or early February.
2. Many tourists enjoy festivals in Viet Nam although they don't understand Vietnamese culture very much.
3. Mrs.Yen forgot to turn off the faucet when she left for work.
4. Ba ate a lot of food although he wasn't very hungry.
5. I am writing to you about the problem of fish catching in the lake near/behind my house.
6. I think local authorities should prohibit and fine heavily anyone catching fish in this way.