Which statement about the economic development of the Swahili coast in SE Africa after 1000 CE is correct?

a. Cities on the coast traded with Arabs, Asians, and others who helped develop more than a dozen port cities that would eventually become major trade centers.
b. People on the Swahili coast had a giant surge in wealth and power resulting from their large gold and iron resources, but once those resources were depleted, development came to a standstill.
c.The cities of the Swahili coast enjoyed a positive trade relationship with Persian and Indian merchants for many years until the Arabs overtook the coastal cities, which ended the economic development.
d.The Swahili coast experienced tremendous growth during that period due to an influx of farmers and cattle herders that brought new resources to the area.