Agricultural Revolution (begins in 18th c.)--Industrial Revolution (begins in late 18th c)--Urbanization--World population hits 1 billion (1804)--Flu Pandemic (1918)
Not sure if "dramatic population growth in developing countries" is a separate choice or part of the population hitting 1 billion. Population growth occurred through this whole time period but particularly after the Agricultural Revolution as it produced better food more cheaply. This boom would allow for more people to be available to work in industry as it develops.
The correct answer is: an extreme political ideology.
Indeed, the movement’s ideology was a very dogmatic blend of several ideologies and outlooks. They were heavily Stalinist and Maoist in their leftism; extremely xenophobic against both foreigners and national non-Khmer minorities and extremely agrarian.
With regards to their Stalinist/Maoist outlook they believed in absolute obedience to the party and its leaders, with a set of inviolable strict rules and laws and the belief that the ends justified any means.
They loathed national minorities as they saw them as a stain on their national Khmer purity and they despised foreigners because they refused to ever be colonized again, whether by Westerners or Asians.
They also considered that urban, capitalistic society was a disease and sought to eradicate it by eradicating its people.
