The correct answer is - B. they needed access to bodies of water for trade.
Sub-Saharan Africa is a vast region that has an abundance of natural resources and a lot to offer for trade for the people of the other continents, thus making a profit for themselves. The region has gold, diamonds, oil, natural gas, as well as lots of spices, fruits, and vegetables. This made the people to start and settle in, and over time to create large cities, along the rivers and the coast, as this were the places where they were able to trade easily and earn more.