Success in trade always deals with advertisement. In this case, the spread of Islam help distribute it's teachings and beliefs making more people contribute to trade.
The correct answer is - B. the ice age pushed animal herds from Asia into the Americas.
During the last ice age there was a big migration of animals from Asia towards North America, and this was made available by the Bering Land Bridge that was connecting the two continents in this period. Because the people in Eastern Siberia were heavily dependent on meat, once the amount of animals started to become critically low they started to move towards North America, using the same passage as the animals before them, the Bering Land Bridge. Even though it was not easy, and they needed multiple tries and expeditions to manage to settle in the new environment, they managed it at the end and became the first humans in the New World.
When people are united by common history and culture, they belong to a "nation" although this is not technically the case. Many people from different backgrounds and cultures can be part of a nation.
