GWD-24-1

Tom Bradley was mayor of Los Angeles from 1973 to 1993, an era when the city had transformed from a collection of suburban neighborhoods to the second-largest city in the United States

A.an era when the city had transformed.

B.an era during which the city was transformed.

C.an era that transformed it.

D.during which era the city transformed.

E.during which the city was transformed.

貌似答案是B。通过transform的形式排除了A,C和D，但是不清楚B前面的an era到底能不能省略（即E的形式）。求教。