Each week,the show took them - and viewers - on an emotional and mind-bending roller coaster ride, through a dizzying loop of supernatural forces, armed conspirators and,of course,romance."

VOA: standard.2010.05.25

So first time through the loop, the answer is a. Second time it-sorry, as it enters the loop, at the time it enter exits a the answer is a.

当第一次完成循环的时候，答案是a，第二次，对不起，在第一次进入循环的时候,答案就是。

麻省理工公开课 - 计算机科学及编程导论课程节选