WLS JMS ack options

[Andrew Ao]

Hi, i just had a sample test for wl 6.0 test from bea. i couldn't understand why the answer to the following question is D.
Which of the following JMS message acknowledgment options minimizes session overhead?
A. NO_ACKNOWLEDGE
B. AUTO_ACKNOWLEDGE
C. CLIENT_ACKNOWLEDGE
D. DUPS_OK_ACKNOWLEDGE
I thought WL has a proprietory acknowledge mode - NO_ACKNOWLEDGE, which doesn't require any acknowledgment after message delivery to client. so, i thought this should have less overhead than DUPS_OK_ACKNOWLEDGE.
thank you,
-andrew