单选题：根据以下材料回答第题：Scientists have come up with a theory forwhy time

根据以下材料回答第题：
Scientists have come up with a theory forwhy time flies when yon are having fun and drags when you are bored．
Scans have shown that (1) of activity in the brain change depending on how we focus on a task．Concentrating on time passing，as we do when bored，will (2) brain activity which will make it seem as though the clock is ticking more (3) ．The research，by the French LaboratoryofNeurobiologyand Cognition．is (4) in the magazine Science．
In the study，12 volunteers watched an image while researchers (5) their brain activity using MRI(核磁共振成像)scans．Volunteers were given a variety of tasks．In one they were told to concentrate simply on the duration of an image．in (6) they were asked to focus on the color，and in a third they were asked to concentrate on both (7) and color．
The results showed that a network of brain regions was (8) when more subjects were paid attention to duration．
It is thought that if the brain is busy focusing on many (9) 0f a task，then it has to spread its resources thinly，and pays less heed(注意)to time passing．Therefore，time passes without us really (10) it，and seems to go quickly．(11) ，if the brain is not stimulated in this way．it concentrates its (12) energies on monitoring the passing of time．This may make time seem to (13) ．but in fact it is probably a more accurate perception of reality． (14) ，the researchers found that the more volunteers concentrated on the duration of the images．the more (15) were their estimates of its duration．
Lead researcher Dr．Jennifer Coull said many of the areas of the brain (16) in estimating time were the same that played a key role in controlling movement．and preparing for action．She said this (17) suggests that the brain may make (18) of time as intervals between movements．in much the same way (19) musicians mark time with his foot．or athletes (20) the sound of a starter’s pistol．
填入（1）处的最佳答案是（　）。