OCP 1Z0 052 27

27. Which two statements are true regarding transactions in an Oracle database? (Choose two.) 
A.Multiple transactions can use the same undo segment. 
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.The  transactions  use  system  undo  segment  to  store  undo  data  if  all  the  segments  in  the  undo 
tablespace are used.  
Answer: AB

见 http://docs.oracle.com/cd/E11882_01/server.112/e40540/logical.htm#CNCPT305

Undo Segments and Transactions

When a transaction starts, the database binds (assigns) the transaction to an undo segment, and therefore to a transaction table, in the current undo tablespace

Multiple active transactions can write concurrently to the same undo segment or to different segments. 

D 不对 
 if the database instance does not have a designated undo tablespace, then the transaction binds to the system undo segment.
而当有undo tablespace又不够时，会报错