But urban roofs are going green.但城市屋頂正在走向綠色。In 1990, fewer than 40% of Americans lived in urban areas.1990年，只有不到40%的美國人居住在城市地區(qū)。It was not until the 20th century that Great Britain became the first urban society in history — a society in which the majority of people live in cities and do not farm for a living.直到20世紀(jì)，英國才成為歷史上第一個城市社會--一個大多數(shù)人居住在城市、不以務(wù)農(nóng)為生的社會。