Erie has 100 000 people, and Pittsburgh has 300 000 - so from the first two Pittsburgh is the bigger one.
Philadelphia has 1.5 million people living in it - it is also the biggest city in Pennsylvania (this is the state where all the cities are) - and this is also the correct answer!
It is not the capital however, as it's Harrisburg.
I don't know if you have to choose only one answer, because answers 2, 3, and 5. It affected throughout the world, and it also reduced communist nations and changed boundaries.
