D) There was a small number of citizens in Athens and a large amount in the United States.
Direct democracy works best in small communities where all citizens can actively participate and engage in decision-making processes. Athens, with its smaller population, was able to practice direct democracy effectively because it was manageable for all citizens to gather in one place and debate and vote on issues. In contrast, the United States has a much larger population, making it logistically challenging for all citizens to participate in direct decision-making on a national level. Thus, the size difference of the citizenry is a key factor in why direct democracy worked in Athens but not in the United States.