"since the sung dynasty had lost the throne and heaven had cut off their sacrifice, the yuan (mongol) dynasty had risen from the desert to enter and rule over china for more than a hundred years, when heaven, wearied of their misgovernment and debauchery, thought also fit to turn their fate to ruin, and the affairs of china were in a state of disorder for eighteen years." -chinese official letter documenting the song and yuan dynasties, translated and published in china and the roman orient: researches into their ancient and mediaeval relations as represented in old chinese records, published 1885 how was trade impacted during the yuan dynasty noted in the passage above?
a) europe was completely cut off from china during this time
b) trade increased as the silk roads were unified under one empire
c) diplomatic envoys decreased due to the fear of the yuan military
d) portuguese explorers first reached indian ports, eventually leading to trade with the yuan