单项选择题

A.It is NOT possible to switch unless no active transaction exist in UNDO1. 
B.It is possible to switch to UNDO2; but current active transactions will abort. 
C.It is possible to switch to UNDO2; current active transactions will be automatically migrated to UNDO2. 
D.It is possible to switch to UNDO2; only current active transactions will continue to execute inside UNDO1.