"The world"s environment is surprisingly healthy. Discuss." If that were an examination topic, most students would tear it apart, offering a long list of complaints, from local smog (烟雾) to global climate change, from the felling (砍伐) of forests to the extinction of species. The list would largely be accurate, the concern legitimate. Yet the students who should be given the highest marks would actually be those who agreed with the statement. The surprise is how good things are, not how bad.After all, the world"s population has more than tripled during this century, and world output has risen hugely, so you would expect the earth itself to have been affected. Indeed, if people lived, consumed and produced things in the same way as they did in 1900 (or 1950, or indeed 1980), the world by now would be a pretty disgusting place: smelly, dirty, toxic and dangerous.But they don"t. The reasons why they don"t, and why the environment has not been ruined, have to do with prices, technological innovation, social change and government regulation in response to popular pressure. That is why today"s environmental problems in the poor countries ought, in principle, to be solvable.Raw materials have not run out, and show no sign of doing so. Logically, one day they must: the planet is a finite place. Yet it is also very big, and man is very ingenious. What has happened is that every time a material seems to be running short, the price has risen and, in response, people have looked for new sources of supply, tried to find ways to use less of the material, or looked for a new substitute. For this reason prices for energy and for minerals have fallen in real terms during the century. The same is true for food. Prices fluctuate, in response to harvests, natural disasters and political instability; and when they rise, it takes some time before new sources of supply become available. But they always do, assisted by new farming and crop technology. The long term trend has been downwards.It is where prices and markets do not operate properly that this benign (亲戚) trend begins to stumble, and the genuine problems arise. Markets cannot always keep the environment healthy. If no one owns the resource concerned, no one has an interest in conserving it or fostering it. fish is the best example of this. The following are the reasons why the world is not a pretty disgusting place EXCEPT ______.

A. price fluctuate
B. technological innovation
C. climate change
D. government regulation