What is true if you want to switch undo tablespaces from the current one， called UNDO1， to a new one called UNDO2？（）
- AIt is NOT possible to switch unless no active transaction exist in UNDO1.
- BIt is possible to switch to UNDO2； but current active transactions will abort.
- CIt is possible to switch to UNDO2； current active transactions will be automatically migrated to UNDO2.
- DIt is possible to switch to UNDO2； only current active transactions will continue to execute inside UNDO1.