D. Daily life is overflowing with mundane mental events. A paper clip gleams amid stacks of documents, a friend’s faceshines like a beacon out of a crowd, the smell of freshly baked bread evokes childhood memories—thoughts and perceptionssuch as these flow by with monotonous ease.
   So it seems, anyway. Yet given what scientists know about how brains work, even the ability to perceive a paper clip on amessy desk represents an extraordinary and mysterious achievement.
31. In Paragraph 1 of this short article D, the author relates our experience of sensory impressions to ____.
(A) a repeated image
(B) a fast-ascending roller coaster
(C) an unobstructed stream
(D) a stagnant swamp