A. Router A is the master virtual router， and Router B is the backup virtual router. When Router A fails，  Router B will become the master virtual router. When Router A recovers， Router B will maintain the role of  master virtual router.
B. Router A is the master virtual router， and Router B is the backup virtual router. When Router A fails，  Router B will become the master virtual router. When Router A recovers， it will regain the master virtual  router role.
C. Router B is the master virtual router， and Router A is the backup virtual router. When Router B fails，  Router A will become the master virtual router. When Router B recovers， Router A will maintain the role of  master virtual router.
D. Router B is the master virtual router， and Router A is the backup virtual router. When Router B fails，  Router A will become the master virtual router. When Router B recovers， it will regain the master virtual  router role.