Here are the completed sentences with suitable articles:

1. Dak Lak is considered the land of **(the)** elephants in Vietnam.
2. This is **(a)** unique custom of this ethnic group.
3. You can see **(the)** moon clearly in the Mid-Autumn festival.
4. My brother doesn’t like **(the)** present I gave him.
5. Did you buy **(a)** pair of trousers yesterday?
6. He can play **(the)** piano well.
7. He is **(the)** 3rd son in the family.
8. All of **(the)** boys were present.
9. We saw **(a)** snake in **(the)** grass.
10. **(The)** less you work, **(the)** less you gain.
11. Despite **(the)** age of 30, he doesn’t want to work to earn money.
12. He hit himself with **(a)** stick.
13. Although it was late, we still went out for **(a)** while.
14. She is **(a)** famous doctor.
15. I saw **(an)** old man in **(the)** street.