Question

One of the factors that contributed to the downfall of both the Inca and the Aztec empires was their

Answer 1

One of the main factors that contributed to the downfall of both the Inca and Aztec empires was their lack of natural immunity to diseases that the Spanish brought with them from Europe. 

Answer 2

The answer is 
a. lack of a natural immunity to diseases that the Spanish brought with them from Europe.