The current data center has run out of space. A room that is located next to the existing data center is then used to house two racks containing six servers. Six months later， physical housekeeping is performed as part of a mid-year maintenance plan. Which environmental condition discovered in the room is LEAST critical？（）

• A、 There are discolored ceiling tiles near the middle of the room.
• B、 Large amounts of dust have accumulated in the corners of the room.
• C、 The anti-static carpet has been removed resulting in a bare painted concrete floor.
• D、 A wet pipe water-based sprinkler system has been installed above the racks.