C. It allowed Germany to transfer troops to France
Think about it- Germany is situated between the powers in the west and Russia to east. Since Germany was fighting both sides, they had to contribute resources to both fronts, therefore fighting a "2 front war". This, of course, is expensive in resources and harder to keep up. WIth the Russian concessions, Russia was no longer fighting Germany; therefore, Germany could move the now unecessary troops from the Russian front to the west.