Which of the following contributed greatly to the fall of the Song Dynasty?

Athe invention of gunpowder
Bthe increase in the Chinese population
Cthe improvement of the civil service system
Dthe vulnerability to invaders from the north

Verified Answer :

It was primarily "d. the vulnerability to invaders from the north", since the Mongol territory was rapidly expanding--making the Mongol warriors all the more powerful. 

D. the vulnerablitliy to invaders from the north. (just took the test)