单选题 Perhaps the most significant postwar trend was the decentralization of cities throughout the UnitedStates，( )when massive highway-building programs permitted greater suburban growth.

A、 and accelerated a phenomenon
B、 a phenomenon that accelerated
C、 accelerating a phenomenon which
D、 the acceleration of which phenomenon

【正确答案】 B

【答案解析】考查同位语结构。a phenomenon是the decentralization of cities throughout the United States的同位语。A项中and为连词，后面应该连接一个完整的句子，但缺主语，故错误。C项语序混乱，which后不能接when引导的时间状语从句。D项中of which作介词宾语，后面应该接一个完整的句子，但只有phenomenon 一个主语，句子不完整，因此错误。故本题选B。句意：可能战后最重大的趋势就是美国城市的分散化，当大规模高速公路建设项目促进了郊区的发展时，这种现象就更明显了。