How did the gold-salt trade benefit ghana

Okay, so. Ghana was inbetween the North and the South. The North had great salt mines and the south had great gold mines. Although Ghana had none of those, it had a great army. So Ghana decided to make a fair deal. The people from the South and the North would get great protection if they each gave an ounce of gold and salt. Although Ghana had none of the mines, it became rich with the salt and gold. 
The king of Ghana took advantage of their location. Since Ghana was located in the middle, everytime foreigners had to go to the other side, they would pass through Ghana. The travelers always had lots of riches with them so the king of Ghana decided that if they wanted to get through then they had to pay him or give him some of their riches. With this he became very rich!
Hope this helped

The answer for your question is A