Have you ever heard of “green roofs”?// They are roofs of buildings that are covered with plants.//
　Green roofs are becoming popular / especially in large cities / because they are good for the environment. //// In summer, / cities are often hotter / than other areas. //// Green roofs keep buildings cool / ①( without ) electricity. //// These buildings produce less CO2 / because they do not use much air conditioning. //// Furthermore, / plants on the roofs / take in CO2. ////
　Green roofs may sound new, but actually they have a long history.// In Norway, farmhouses have had grass roofs for centuries.// In Japan, people have had straw-roof houses ②(    for    ) thousands of years.// These traditional roofs show ancient people’s wisdom.//
　In many cities, the natural environment has been replaced ③(    by    ) buildings.// Green roofs may be an effective way for people in cities to live in harmony with nature.//