Answer:
A. To keep invaders out of China's walls
Explanation:
I believe it was because they owed money
Mongol leader Genghis Khan (1162-1227) rose from humble beginnings to establish the largest land empire in history. After uniting the nomadic tribes of the Mongolian plateau, he conquered huge chunks of central Asia and China. His descendants expanded the empire even further, advancing to such far-off places as Poland, Vietnam, Syria and Korea. At their peak, the Mongols controlled between 11 and 12 million contiguous square miles, an area about the size of Africa.
