单选题
Perhaps the most significant postwar trend was the decentralization of cities throughout the United States, ______ when massive highway-building programs permitted greater suburban growth.
【正确答案】
B
【答案解析】解析：句意：可能战后最重要的趋势就是美国城市的分散化，当大规模的高速公路建设项目让郊区获得更大发展时，便加速了城市分散化这一现象。本题考查同位语结构。aphenomenon是the decentralization of cities throughout the United States的同位语。when引导时间状语从句。因此B项正确，that accelerated定语从句，修饰the phenomenon。A项中and后面的内容与前半句无法构成并列关系，因此A项错误；C项中which后面缺少定语从句；D中phenomenon为多余成分，且of which后缺少定语从句．