"In the whole scheme of things, green roofs are not the one answer to sequestering carbon, but they will certainly help." Green roofs offer a long list of known benefits. They lower air-conditioning costs in the summer by absorbing and reflecting heat. They lower heating costs in the winter by adding extra isolation.Green roofs appeal to cities because they soak up rainwater, making excess storm-water less likely to flood sewage(下水道) systems and increase sewage treatment costs. Plant-filed roofs make urban areas less likely to become heat islands. They reduce air pollution and noise pollution. And plants, even when it's several stories up, provides living places for animals.50. What's the main idea of the passage?

Filing roofs with plants and dirt can help pull a modest amount of heat-trapping carbon dioxide(二氧化碳) out of the atmosphere, found by a new study. While green roofs certainly won't solve the global warming problem, their ability to absorb greenhouse gases—even just a little bit—supports the case for planting them on city buildings, despite extra costs on the front end, said lead researcher Kristin Getter, of Michigan State University in East Lansing. "The key to fighting global warming is capturing carbon from the atmosphere and storing it in new reservoirs that weren't storing carbon before," Getter said. "In the whole scheme of things, green roofs are not the one answer to sequestering carbon, but they will certainly help." Green roofs offer a long list of known benefits. They lower air-conditioning costs in the summer by absorbing and reflecting heat. They lower heating costs in the winter by adding extra isolation. Green roofs appeal to cities because they soak up rainwater, making excess storm-water less likely to flood sewage(下水道) systems and increase sewage treatment costs. Plant-filed roofs make urban areas less likely to become heat islands. They reduce air pollution and noise pollution. And plants, even when it's several stories up, provides living places for animals. 50. What's the main idea of the passage? A. How to fight the global warming problem. B. The benefits and cost of a green roof. C. A green roofs ability to absorb carbon dioxide. D. A green roofs ability to soak up rainwater.