Which statement is true about undo retention?

In your database instance, the UNDO_RETENTION parameter is set to 1000 and undo retention is not
guaranteed for the fixed size undo tablespace.
Which statement is true about undo retention?

A.
Undo is retained in the UNDO tablespace for 1000 seconds, and then moved to the SYSTEM tablespace to
provide read consistency.

B.
Inactive undo is retained for at least 1000 seconds if free undo space is available.

C.
Inactive undo is retained for 1000 seconds even if new transactions fall due to lack of space in the undo
tablespace.

D.
Undo becomes expired obsolete after 1000 seconds.

Explanation:
https://docs.oracle.com/cd/B19306_01/server.102/b14231/undo.htm