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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.

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出自：Oracle认证

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