According to Edward Glaeser, what is one effect of large concentration of people in a small area?

Edward Glaeser is high on cities. Very high, in fact. In “How Skyscrapers Can Save The City” (The Atlantic, February 2011) the Harvard economist puts the high-rise at the heart of a newly accessible, affordable, vital and sustainable metropolis. The city that doesn’t build up must build out, Glaeser points out, sucking up resources, lengthening commutes and putting pressure on undeveloped land. He sees densely populated, vertical cities not only as environmentally responsible, but as engines of innovation and prosperity — and the best hope for developing nations. Yet in cities around the world, Glaser’s lofty vision has bumped up against height limits and restrictive permitting. He spoke with The Atlantic from his home outside Boston about how measures aimed at saving cities may actually threaten their survival.

The answer would be d, coherence.