In Terms of Their Impact, What Set Marx and Engels

In terms of their impact, what set Marx and Engels apart from other socialist and communist intellectuals?

A) They tried to suppress trade unions because they thought unions would slow the advance of socialism.
B) The Communist Manifesto became the foundation document for communist revolutions around the world.
C) They were willing to work with liberals and other reformers in England and Germany.
D) They put religion, and particularly early Christianity, at the heart of their ideology.