1. ______is now being questioned by more and more people.
2. Although many people claim that, along with the rapidly economic development, the number of people who use ______are decreasing and ______is bound to die out. The information I've collected over the recent years leads me to believe that ______will continue to play extremely important roles in modern society.
3. experts point out that increasing______not only causes serious problems such as global warming but also could threaten to end human life on our planet.
4. In view of such serious situation, environmental tools of transportation like______are more important than any time before.
5. Using______contributes greatly to people's physical fitness as well as______