主题：【文摘】一道有趣的概率题 -- 天下第一银杏树

This can be better understood by drawing a Venn diagram.

In your interpretation, the case might as well be presented as follows: The hostess first pick a door randomly, which happens to reveal a goat. Conditioned on this, would you have any preference as to which of the remaining two doors to pick? Of course not, since each door would have the probability of 1/2 of hiding a car!