Which of the following best explains a cause of the economic activity described in the passage? the large-scale migration of Indians into western China
the outbreak and spread of the bubonic plague
the unification of the Eurasian trading network under the control of China’s economic system
the increase in long-distance trade as a result of Mongol-protected Silk Roads

"Refer to the passage.

Furthermore all merchants arriving from India or other countries, and bringing with them gold or silver or gems and pearls, are prohibited from selling to any one but the Emperor. He has twelve experts chosen for this business, men of shrewdness and experience in such affairs; these appraise the articles, and the Emperor then pays a liberal price for them in those pieces of paper. The merchants accept his price readily, for in the first place they would not get so good a one from anybody else, and secondly they are paid without any delay. And with this paper-money they can buy what they like anywhere over the Empire, whilst it is also vastly lighter to carry about on their journeys. And it is a truth that the merchants will several times in the year bring wares to the amount of 400,000 bezants, and the Grand Sire pays for all in that paper.

Excerpt from The Book of Ser Marco Polo, the Venetian, Concerning Kingdoms and Marvels of the East, Vol. 2, 1871"