As far as I am concerned Gao was not one of the Songhay Empire's major ones but I bet two answers should be acceptable in this question. I also think that you can include the third option from the scale represented above because <span>Algiers were quite far away on north so they couldn't be the part of the Songhay. </span>
Oil maybe I think it is about oil
The answer to your question is cotton
hope this helps
The Silk Road was the easiest way for people to trade supplies but over the years people started to taxes everything in the silk road to the point no one wanted to do any trading there
