A method and apparatus for supplying oxygen to meet a user demand in which oxygen is separated from air by an electrically driven oxygen separation device to supply oxygen to meet the user demand and to charge an adsorbent bed with all or part of the separated oxygen. Such methods and apparatus may be based upon an allowable capacity of presentations storable in device memory, as well as on a required memory associated with each presentation in a media service. The guide unit is configured to support detachably the sheet tray. At least a portion of the TRS that obstructs the passage abuts the first opposed wall. Thus leakage through unselected cells is low so power wasted is small, and assurance is high that no unselected memory cells are disturbed. This calculated time will be the amount of time in the speed round for that particular contestant or team.