What is true if you want to switch undo tablespaces from the current one, called UND01, to a new one called UND02?

What is true if you want to switch undo tablespaces from the current one, called UND01, to a new one called UND02?

A.
It is NOT possible to switch unless no active transaction exists in UND01

B.
It is possible to switch to UND02,but current active transactions will abort

C.
it is possible to switch to UNDO2; current active transcations will be automatically migrated to UND02

D.
it is possible to switch to UNDO2; only current active transcations will continue to execute insideUND01

Explanation:
The switch operation does not wait for transactions in the old undo tablespace to commit. If there are any pending transactions in the old undo tablespace, the old undo tablespace enters into a PENDING OFFLINE mode (status). In this mode, existing transactions can continue to execute, but undo records for new user transactions cannot be stored in this undo tablespace.

An undo tablespace can exist in this PENDING OFFLINE mode, even after the switch operation completes successfully. A PENDING OFFLINE undo tablespace cannot be used by another instance, nor can it be dropped. Eventually, after all active transactions have committed, the undo tablespace automatically goes from the PENDING OFFLINE mode to the OFFLINE mode. From then on, the undo tablespace is available for other instances (in an Oracle Real Application Cluster environment).

REF: Oracle(r) 10g Administrator Guide, 10-9