Which of the following caused internal conflict during the yuan dynasty?

There were essentially two factors that caused internal conflict and eventually led to the downfall of the Yuan dynasty:1. The class conflict caused by the heavy taxation of the classes. 2. There was ethnic conflict due to the caste system of the day. 
The four classes of people were, in ascending order, the people from Southern China, the Han people (Northern China), the Semu people, and the Mongolian people. The Mongolian people were considered the highest class because they conquered China during this time. This class system also affected the way the people were taxed. The people from Southern China were taxed the most and the Mongolians were taxed the least.

What are they! i have no idea lol