Answer: B. Spread of the bubonic plague.
Step-by-step explanation:
The spread of the Bubonic Plague or the Black Death has nothing to do with the Mongols. Scientists speculate that the first wave of the disease arrived in Europe by sea trade routes from Asia to Italy. Italy has been a center of European trade for a long time, and the diseases were transmitted to the mainland by infected ship rats. The plague ravaged Europe on several occasions, and in the first wave, half of the European population died as a result of the infection.