Which of the Following Is One of the Reasons for the Spread

Which of the following is one of the reasons for the spread of democracy in the world?

A) Many totalitarian regimes failed to deliver economic progress to the vast bulk of their populations.
B) The rise of dominant nuclear powers.
C) The failure of post-communist countries to achieve economic growth.
D) In many countries,the economic downturn of the past quarter century have led to the decline of prosperous middle and working classes.