pp语法

4. Perhaps the most significant postwar trend was the decentralization of cities throughout the United States, _________ 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

选B，不明白为什么不是that is accelerated,

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容许了更大程度的发展郊区，这个现象更被促进了

ACCELERATE can be both vt.and vi.