How can i study and learn history? i can't remember all

How did the russian revolution impact world war i? a) the russians required the allied powers to come to their aid and defeat the radical groups. b) the russians pulled out from the war, enabling the germans to focus on the western front. c) the russians created new technological advancements that were used by the allied powers to win. d) the russians created a new communist government that aligned with the central powers instead.