Here are the completed sentences with the appropriate comparative or superlative forms:

1. Her dress is **prettier** than mine.
2. Cheetahs are **the fastest** animals we can find.
3. Eating fruit and vegetables is **healthier** than eating hot dogs.
4. I like milk **better** than coffee.
5. China has **more** people than any other country in the world.
6. The blue whale is **the heaviest** animal in the world.
7. Which is **bigger**, Portugal or Spain?
8. Travelling by plane is **more comfortable** than travelling by car.
9. He is **the untidiest** person in class.
10. Buying things from plastic is **worse** than buying things from recycled paper.
11. The Nile is **the longest** river in the world.
12. In Mark’s opinion, History is **more difficult** than Geography. However, it is **more interesting**.
13. Antarctica is one of **the coldest** places on Earth.
14. Greece is **hotter** than England.
15. Mirror, mirror on the wall, who’s **the prettiest** of them all?
16. Elvis Presley is one of **the most popular** rock singers ever.
17. Jenny is **taller** than Alice even though she is **younger**.
18. Switzerland is one of **the richest** countries in the world.