The Mongols invaded Korea, known as Goryeo at the time, on several occasions between the years of 1231 and 1270. Which
statement is true of those invasions?
A. The Mongols were forced to retreat by the strong Korean army during every campaign.
B. The Mongols lost nearly all of their soldiers, but there was minimal loss of civilian life.
C. Korea became a vassal state of the Mongol Yuan Dynasty for nearly 80 years.
D. Korea defeated the Mongols with the help of the Japanese and the Chinese.