Which of the following contributed greatly to the fall of the song dynasty?
a. the invention of gunpowder
b. the increase in the chinese population
c. the improvement of the civil service system
d. the vulnerability to invaders from the north

  D. The vulnerability to invaders from the north.

Explanation:

  Initially, the Song Dynasty had an alliance with the Mongols to maintain the north calm. Both tribes had the same enemy, the Jin Dynasty. But after the Mongols defeated Jin´s the alliance broke up because the Song captured the former imperial capital of Kaifeng. The Mongols then started a campaign against the Song that lasted 20 years and ended up with the creation of the Yuan Dynasty by Kublai Khan.

  I hope this answer helps you.

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