U2R1 P3:This unprecedented movement of people,unimaginable only a century ago, is forecast to continue and intensify in the next few decades.这一史无前例的人口运动是100年前的人们无法想象的，预计这一趋势在未来的几十年将持续和加强。U2R2 PB:While the world's urban population grew very rapidly (from 220 million to 2.8 billion) over the 20th century, the next few decades will see an unprecedented scale of urban growth in the developing world.尽管20世纪的世界城市人口增长速度非常迅速（从22亿到28亿），未来的几十年里还将会见证发展中国家城市发展的空前规模。All of us have witnessed ____________________, which ____________________(史无前例的经济发展，这在上个世纪是难以想象的).