What is true if you want to switch undo tablespaces from the current one， called UNDO1， to a new one called UNDO2？（）

• 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.