could have done 和 might have done 的区别?

题目

could have done 和 might have done 的区别?
有一个题：
-Believe it or not ,yesterday I drove home after drinking 5 cups of beer.
-Oh my goodness ,you _____have taken a taxi.
A.might B.could
正选为B 为何不能选A呢?

答案

,他说他昨天晚上喝了5杯啤酒后开车回家,另一个说 ,天啊,你应该坐车回来 ,might ,为可能 ,用在这不合适 .