How did the Mongol Empire's rise to power affect the Silk Road?

A. The Mongol Empire purposefully spread the Black Death on the Silk Road.
B. The Silk Road was very stable during Mongol rule.
C. The Silk Road was very unstable during Mongol rule.
D. The Mongol Empire closed the Silk Road because of threats to their power.

C. The Silk Road was very unstable during Mongol rule.