Our environment is getting worse and worse with the increase of the world population, which affects the environment in two ways. Firstly, the limited energy resources will be used up much faster. Secondly, the increasing population creates more pollution, another severe problem that needs to be solved. Both problems are long-term ones because action taken now shows their results slowly over many years. They are also urgent because delays in action can lead to great suffering and social problems. The question seems to be difficult to handle for most people. However, a person can be a protector of environment in everyday life if he takes action to save the environment right now. With the development of technology, cars make transportation easier and quicker than before, but we can't ignore the disadvantages brought to us by more and more cars in use. For example, we are consuming gasoline (汽油), which is a non-renewable (不可再生的) resource, and will soon be gone. Moreover, the exhausted gas from cars pollutes our air, and our health is therefore threatened by air pollution, which has a big potential effect on our daily life. So we can consider walking, taking buses, carpooling (拼车), instead of driving cars alone. This seems to be a very slow process that can't be so effective if it's taken by a single person. But when more and more people become aware of the importance and positively take it as their personal responsibilities, the condition of the air will be improved to a great extent.
单选题
According to the writer, the main problem to our environment today is______.