What Accounted Most Lastingly for the Early and Rapid Success

What accounted most lastingly for the early and rapid success of the conquistadors against Native Americans?

A) the military technology of the Spanish
B) the infectious diseases brought by the Spanish
C) the rigid political centralization of the Aztecs, which meant that to capture the emperor was to conquer the empire
D) the bloody religious system of the Aztecs, which meant that the Spanish stress on Christian virtue won converts among Indian peasants