GMAT Sentence Correction (SC) | Evidence suggests that the people who crossed the Bering Strait

I would like to understand the difference between A and B in detail.

A) nomads living in an egalitarian society where they obtained their sustenance almost entirely from

B) nomads living in an egalitarian society that obtained its sustenance almost entirely from


egalitarian society --> a society where everyone is treated equally. This will act as a collective noun and will be singular and hence its sounds correct. B seems more refined than A?

But with the explanation that --> society