Which of the following best explains a cause of the
economic activity described in the passage?
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.
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

Some answers may be different but from when I took the test, D was the correct answer. If you think about the previous answer, the correct answer was the growth of banking houses. What effect did the growths of banking houses did it have on the economic activity? Migration didn't effect economic activity, it only affected the population. B is not right because the bubonic plague would of decreased the economic activity and banking houses made it increased economic activity. Where in the passage does it say about unification had to do with economic activity? So C is not correct. D is only correct because banking houses led to long-distance trading and the development of interregional trade of luxury goods.