A、It’s because there are many developing nations. B、It’s because people use too many man-made materials. C、It’s because we have

Our environment is being polluted faster than nature and man’s present efforts can prevent it. Time is bringing us more people, and more people will bring us more industry, more motor vehicles, larger cities, and the growing use of man-made materials.
    What can explain and solve this problem? The fact is that pollution is caused by man—by his desire for a modern way of life. We make "increasing industrialisation" our chief aim. For its sake, we are willing to sacrifice everything: clean air, pure water, good food, our health and the future of our children. There is a constant flow of people from the countryside into the cities, eager for the benefits of modern society. But as our technological achievements have grown in the last twenty years, so pollution has become a serious problem.
    Isn’t it time we stopped to ask ourselves where we are going and why? It reminds one of the story about the airline pilot who told his passengers over the loudspeaker: "I’ve some good news and some bad news. The good news is that we’re making rapid progress at 530 miles per hour. The bad news is that we are lost and don’t know where we’re going. "The sad fact is that this becomes a true story when applied to our modern society.