选词填空。
school   cars   easy   air    exercise    use
Many people in the world ride bicycles every day. There are more bicycles than ________ in the world today. A lot of Chinese people ride bicycles to go to work or ________. Bicycles are much cheaper than cars. And it is ________ for people to learn to ride. Bicycles are also used for ________. It helps us to keep fit. If each family has a car, it will ________ a lot of energy and pollute the ________. So let us ride bicycle.