The correct answer is - D. transporting goods over long distances.
The river civilizations were all able to produce much more food than what they needed. Also, because the food problem was solved, they had the time to produce lot of other goods, be it metalwork, glass objects, colors etc. Since they had large navigable rivers at disposal, they used them as an easy transportation mechanism for longer distances. By being able to transport the surplus food and other goods over longer distance, these civilizations developed the trade, and were becoming more and more wealthy. The wealth was used for building infrastructure, sacred places, the military, so the civilizations were both becoming stronger but also were making the others to be dependent on their goods.
