More than half the world's population now lives in cities. The populations of cities are getting bigger and pollution is getting worse. So which cities are trying to solve this? We asked you which cities you think are the greenest. 
Calen:
I chose Barcelona because I (1) _____ it's a great example of a green city. There are lots of buses here and people often ride their bikes to school or walk. There is also a metro system and cable cars. I think that the pollution from cars is a really big (2) _____ , so Barcelona is a green city because it encourages people to take other forms of (3) _____  instead of their cars. Also, there are lots of parks in the (4) _____  of the city because it strongly believes in keeping the city green and healthy. The parks have lots of grass and trees and the air (5) _____  is very good. People (6) _____  to them to relax, to have lunch or to play sports. 
Winston: 
Singapore is a green city in Asia. I think something like 30% of the city is covered by trees and green spaces. The trees help to reduce the hot temperatures and noise from traffic, giving people who live in Singapore a better quality of life. Even the airport is green because there are gardens and a butterfly farm. That's so smart! In the past, there was a lot of pollution but now Singapore wants to be famous for being green. I found a website about green things in Singapore and things you can do. You can pick up the rubbish in your area with your friends. I think it's great and people learn that it's not good to leave rubbish on the street. 
Poppy:
Bogotá in Colombia is an interesting city because in 2016 it had a week where no cars were allowed into the city. There were buses but not many taxis. The city has also had 'Cycle Sunday' for over 30 years. The roads become bicycle paths. I think that's really cool because then you really think about the environment. The idea is that there is less pollution and everyone has to get into town in a 'green' way - walking, cycling, or bus. The idea was so successful that the mayor decided to create 300 km of bicycle lanes, which increased bicycle use by five times in the city. There's a good reason for it being called 'the greenest city in Latin America'! 
Kelly:
 
I didn't think Australia had a lot of pollution, but it does. Adelaide in South Australia is a green city. There are 29 parks and there's a big hotel there which recycles its energy and water. Also, there are a lot of cycle paths and you can hire a bike for free. That's really good. There is a bus that doesn't use petrol: it is solar powered - you know, it uses energy from the sun! That's awesome! 
 A. Talking about green cities around the world 
 B. Pollution around the world is getting worse
 C. What have you done to make the Earth healthier?