@Shirnel I'm not an instructor but I think a key distinction in the passage is what makes (B) the right answer. There is city G and city R, and then there is "central" G and "central" R.
The passage states that city G has a larger population than city R. But what matters most is the location of this population. Is the population more dense in central G than central R? Or is most of city G's population outside of the city's center?
(B) suggests that central G is more heavily populated than central R. This is important because the population that is expected to cause the increase in library use must be within walking distance.
