For many years scientists have said that the earth is getting warmer. Now they have found something new-hot cities! In the southern US, they have found that cities become very hot in summer. For example, the city of Atlanta in Georgia has roof  temperature of up to 50oC while it is 27degC in the streets! At night, the outside of buildings stays so hot that the heat of the city causes storms over the city! China also has the same kind of problems. Every year more farmland is used for factories or offices. More housing is needed, too. As people become richer, they buy more cars. New roads are needed and new car parks are built in the city centers. This all makes cities hotter. It isn't easy to change the situation. City planners say that we should plant more trees in the middle of cities. Every new street should have trees on both sides, they say. Trees make the temperature lower, so we should have more trees in our parks and squares. We should also paint our roofs white. If we do this, they do not become so hot. Every roof in a hot and sunny country should have solar  roof panels. The electricity from these can be used to run the air conditioners  in the buildings. People continue to cut down forests around the cities. This makes the problem worse. Cities are growing faster and faster. By the year 2025, four fifths  of the population will be living in cities. If we go on like this, there will not be enough farmland to feed everyone in the world The way to make the city cooler is to  _  .
plant more trees try to use air conditioners more build more parks and squares paint the roofs of all the white buildings