A、More vehicles. B、More coal-powered plants. C、The aging population. D、The growing middle class. C录音提到，研究指出即使污染水平保持不变，印度和中国的死亡率也

Air pollution is the biggest environmental health risk in the world, says the World Health Organization. Part of the problem is particulate matter, very small particles floating in the air. Particulate matter, also known as PM, may get trapped in the lungs. Scientists have long known about the harmful effects of breathing PM. The particulates are linked to an increased risk of breathing problems, heart attacks, strokes and lung cancer.
    A U.S. based team has been studying the issue. Its members are environmental engineers and public health researchers. The team recently prepared a model to show how much people would be helped if air pollution from PM is cut to the WHO’s suggested limits. Joshua Apte is an assistant professor of environmental engineering at the University of Texas, Austin. He led the study. "About 75 percent of the total number of deaths that could be avoided from cleaner air could come from improving air in some of the most polluted parts of the world, like India and China." He adds that improving air quality could bring as many health benefits as when we take steps to deal with major diseases like AIDS.
    The study estimates that reducing air pollution around the world could save 1.4 million people in places like India and China. But that is not an easy goal. The report says that even to reduce mortality rates in half, India and China would need to cut pollution levels by almost 70 percent from their 2010 levels. In places like New Delhi, PM levels are sometimes 10 times higher than the amount set in the WHO’s air quality guidelines. India and China are home to some of the world’s most polluted cities. Thirteen of the world’s 20 most polluted cities are in India. The capital, New Delhi, is at the top of the list. The study also shows that even if the pollution levels stay the same, death rates would jump by over 20 percent in India and China.
    Joshua Apte explains that the main reason for this increase is population changes in these countries over time. He notes that as the population gets older over time, the number of people at risk of heart attack or stroke goes up. But it may be difficult for India and China to improve air quality since both countries still use coal for electricity. Many roads are becoming filled with vehicles owned by the growing middle class. China has done some work in cleaning the air. But experts believe that air pollution could worsen in India. They worry that India wants to increase the number of coal-powered plants to fuel the country’s economy. They expressed regret that Indian officials have taken little action to limit air pollution.
    22.What do we learn about particulate matter?
    23.For what did the research team prepare a model?
    24.What do we learn about New Delhi, according to the study?
    25.What mainly causes death rate to increase in China and India over time?