The Pony Express was an ingenious system for carrying mail, having existed only briefly, before the tellegraph system made it obsolete.红色部分是错的，大家说是having existed指代不明，但是 在这种结构里面S+V+O，ving 指代的不是都是主语么？那指代的的就是pony express，for carrying mails是目的状语，不影响啊，，，百思不得其解，望大家解答，麻烦了

解析：

这个答案好像B,mail, for it was in existence only briefly,however,

分开分析吧

情况一：就近修饰，逻辑主语，即只能指mail，错误

情况二：修饰状态，什么状态呢？The PE was a system,这怎么修饰嘛，逻辑~~~~而且having done的用法好像大多用在逻辑主语里面，~