Which two statements are true regarding transactions in an Oracle database?

Which two statements are true regarding transactions in an Oracle database? (Choose two.)

A.
The transactions use system undo segment to store undo data if all the segments in the undo tablespace are used.

B.
A transaction is assigned an undo segment when it is started.

C.
More than one transaction cannot share the same extent in the undo tablespace.

D.
Multiple transactions can use the same undo segment.