Answer:
Here are the sentences completed with the comparative or superlative form of the words in brackets:
The population of the world is bigger (+ big) than it was ten years ago.
Some cities are more populated (+ populated) than others.
Travelling by car is more expensive (+ expensive) than travelling by bus or train.
What's the most beautiful (+ beautiful) place you've been to?
The cheetah is one of the fastest (+ fast) animals in the world.
Sea levels are getting higher (+ high) because the ice is melting fast.
Some scientists say that the Amazon River is the longest (+ long) river in the world.
They're cleaner (-pollute) now than they used to be.
Junk food is cheaper (+ cheap) than healthy food, but it's also unhealthier (- healthy).
Step-by-step explanation: