Trans-Saharan Trade Routes Practice Test

A museum label notes that salt mined in the Sahara could be traded in West Africa for its weight in gold in some periods. The label explains that salt was essential for diet and food preservation in hot climates, while gold was plentiful in some regions but difficult to transport in bulk. Which economic principle is best illustrated?

Between 1000 and 1500, West African rulers often converted to Islam while many rural subjects retained local religious practices. Merchants in caravan cities built mosques, used Arabic for contracts, and relied on Islamic courts to settle disputes. Which factor best explains why Islam spread through trans-Saharan trade cities first?