Why are the corresponding costs often less obvious in life than in the experiments? (Paras.8-9)

A、 Because in life no pictures show you the closing of the doors.;
B、Because in life the doors are closing slowly, not like the ones on the computer in the experiment, so people don’t really see the opportunities actually vanishing away little by little. ;
C、Because in life you can have a real key to the door at hand.;
D、 Because in life no money is behind a door.