Was ist die Bevölkerungszahl Wiens ungefähr?

Why this is the correct answer

Vienna is home to approximately two million people within the city limits, making it by far Austria's largest city and accounting for roughly a quarter of the country's entire population. As a city-state (Bundesland), Vienna functions simultaneously as a municipality and as one of the nine federal states. Its population density contrasts sharply with the rest of Austria, which is predominantly rural.