What Enduring Obstacle Prevented the Development of a Free, Mobile

What enduring obstacle prevented the development of a free, mobile labor force even after Alexander II's Great Reforms?

A) The policy of lengthy military conscription that kept men in the army for twenty-five years
B) The lack of sufficient infrastructure, especially roads and rail transport
C) The fact that the zemstvos often denied former serfs' requests for permission to move
D) The fact that former serfs remained tied to a system of communal landowning and decision making