Took me all of 30 seconds............
C)
Failure to Occupy Space - If a Passenger fails to occupy space which has been reserved for him/her on a flight of UA and UA fails to receive notice of the cancellation of the reservation before the departure, or if any carrier cancels the reservation of any Passenger, UA may cancel all reservations (whether or not confirmed) held by such Passenger on the flights of UA or any carrier for continuing or return space, provided UA or an authorized agent of UA originally reserved that space.
Page 10, para. C