可以扮演的角色有主域控制器（PDC）、备份域控制器（BDC）、独立的服务器，请先简述一下三种角色的功能，再判断本服务器应选择的角色。-could play a role as a primary domain controller (PDC) and backup domain controller (BDC), independent server, please briefly describe the role of the three functions, then judge shou