听力原文：Environmentalists say pollution is terrible and becoming worse. Businessmen retort that a real cleanup would cost too much. The ordinary citizen believes both: the environment is indeed going down the drain, but so might his job if anyone tries to stop that. All of them, it turns out, are wrong. Recent statistics show that, in rich countries at least, many of the worst pollution problems are far smaller than they used to be — and that the costs of this success have so far been small. Start with air pollution. The dreaded greenhouse gases are still flowing into the atmosphere. But output of gases that attack the ozone layer is well under control. Production of CFCs used in refrigerators has fallen by two-thirds since the mid- 1980s. Many pollutants have been beaten back. The flow of oxides of sulfur, which contribute to evils such as acid rain and smog, has fallen by around a third in rich countries since 1980. Emissions of lead, soot and carbon monoxide—each hazardous to human health—have also fallen sharply. Water quality has improved in many ways. There is still worry about chemicals, such as fertilizers and heavy metals, draining off the land. But many of the organisms that once infested rich-world waterways — and which still kill millions in poor countries — are retreating steadily. Progress is due largely to the spread of waste water treatment. Has this cost jobs? There is no evidence for that. Spending on pollution control amounts to 1% — 2% of GDP in most rich countries, but that has not cut jobs overall. Question ：What is this passage mainly about? (30)