I believe your answer would either be B. Suburbs caused people to live with others of the same socioeconomic status or C. suburbs caused American cities to grow rapidly. For B., I think this answer would be correct because a suburban area is an outlying district/residential area of a city and people in the same class would likely live there, most probable to be of the lower class (see "neighborhood," if needed to be cleared up). For C., suburbs are outlying districts, or neighborhoods, in a city. With more suburban areas, American cities will grow rapidly, even if they're made up of the lower class. With low-class neighborhoods making up most of the city, they may excel and become equal with the rest in the city, or at least not so far under.
Mostly, I would have to go with answer choice B.