Answer: "Philadelphia and New York City were the most populated cities in 1860 and would soon reach one million residents."
Explanation: This sentence stands out as it is a detail that is a bit more loosely related and can be removed without affecting the overall flow of the passage. This is an unnecessary detail and sounds odd as the author already mentioned that cities were growing quickly, so it is not necessary and the rest of the passage still sounds good without this part and still is interesting.
