Explanation
Correct answer: C.
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.
Why the other choices are wrong
Choice A
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
Choice B is incorrect. The sentence emphasizes the population sizes of the two countries; it fails to mention the capitals.
Choice D
Choice D 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.