Rome was the first city to reach one million people.
Why it is true
Rome reached a population of one million in 133 BC, during the Roman Republic. London only reached that milestone in 1810, and New York in 1875. Today, over 300 cities have more than a million residents.
Source: History
Would you rather be asked first?
This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.