[A] The Scrum Master has no authority over Scrum team members, but that authority is granted by the team.
[B] If needed, the Scrum Master will work with the Product Owner to make sure the Product Backlog is in good shape and ready for the next sprint.
[C] The Scrum Master role is commonly filled by a former project manager or a technical team leader, but can be anyone.
[D] Changing Scrum Masters midstream a project will encourage self-empowerment of the Development Team.