The correct answer is Trade from Songhai was done by land, while trade in Mogadishu was done by boat.
Trade was an important component of history in West Africa because it allowed the rulers to build great empires, which was the Case of Mobasawa and Kongo.
At the kingdom of Songhai, the major export was gold. Strong armies ride with the caravans by land, to secure trade routes. At the kingdom of Mogadishu, the trade was exporting slaves and ivory, and trade was done by boats. Trade routes were established in the northern and western kingdoms, and from there, they exported to all the continent.