The correct answer is
the first option – Mongol attackers from the north ended the Song dynasty. The
Song dynasty made an agreement with Ögodei Khan, leader of Mongols, to bring
down together the Jin dynasty, but generals of the Song dynasty broke the alliance.
Breaking the alliance enraged Ögodei Khan which resulted in ending the Song
dynasty and slaughtering of over a million people that inhabited the city of
Chengdu.
