Why were the specific targets of the September 11 attacks selected by the terrorists?

The September 11 attacks targeted the Pentagon, World Trade Center and U. S. Capitol Building. Each of these three represent the powers of USA in terms of military, economy, and politics, respectively. The attacks were made to make a significant impact to the people that even though USA could be one of the most powerful countries in the world, they could still be vulnerable to the hands of the terrorists. Also, their objective is mass casualties. Naturally, they would choose places with high volume of the population.