Tough one! I would go with B; anyone can have opinions on anything, so there goes C as an answer. A and D are inherently the same and is the opposite of what debatable means.
The Chinese used water routes and silk could not travel over water.
The Mongols were devastating Chinese territory at this time and the trade routes to sea were dangerous. There was still a high demand for silk, but overland took longer
It is a partnership that is limited.