The long-term impact on the Southern Chinese people of the four social class system was that it opened up trade on the Silk Road and created a new merchant class.
Kublai Khan was the grandson of Genghis Khan and a ruler of the Mongol Empire for over 30 years. He was the founder of Yuan dynasty in present day Mongolia and China.
He was the first Mongol to rule over China when he conquered the Song Dynasty of southern China in 1279.
Thus, Option C is true, as four social class system opened up trade on the Silk Road and created a new merchant class.
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