In my world, a * * * has three floors. The first floor is a big garden, and the ground is covered with pebbles, so that people will be comfortable to walk on it, and it is convenient for people to walk here.
The second floor leads to houses, schools, shops, restaurants and other buildings, and there will be no traffic jams.
The third floor is to go to the countryside and some distant places, such as Germany, France, Italy, Belgium and so on. Usually parents will drive their children to far away places to play. I think it pollutes the air very much. Do you know why? Because there are two round things under the rear of the car, called exhaust gas. Let me introduce you to what's in the waste gas! Scientific analysis shows that there are hundreds of different compounds in automobile exhaust, among which pollutants are solid suspended particles, carbon monoxide and other compounds. Did you understand after listening to my introduction? Have you accumulated some knowledge?
In my world, cars don't run on the ground, but fly in the sky, so they won't pollute the air!
How, my world is very good!