OG详解-OG2 文法 Q12

Choice C is the best answer. The sentence emphasizes the relative sizes of the capital cities’ populations, noting that even though Hanoi has more people overall, Ulaanbaatar accounts for a larger percentage of the people in its country. 
Choice A is incorrect. While the sentence indicates the population size of each capital, it fails to emphasize their sizes relative to each other or to their countries’ overall population sizes. Choice B is incorrect. While the sentence indicates the population size of each capital, it fails to emphasize their sizes relative to each other or to their countries’ overall population sizes. Choice D is incorrect. The sentence emphasizes the population sizes of the two countries; it fails to mention the capitals