The brass needed to make the plaques was usually transported in the form of large bracelets – called manillas – and the quantities involved are staggering. In 1548 just one German merchant house agreed to provide Portugal with 432 tons of brass manillas for the West African market. When we look again at the plaque, we can see that one of the Europeans is holding a manilla, and this is the key to the whole scene: the Oba is with his officials who manage and control the European trade. –A History of the World in 100 Objects,
Neil MacGregor

What central idea is best supported by details in the passage?