请教SAT语法
The Pony Express was an ingenious system for carrying mail; it was in existence only
briefly,however,before the telegraph system made it obsolete.
(A) mail; it was in existence only briefly,however,
(B) mail,for it was in existence only briefly,however,
(C) mail; however,existing only briefly
(D) mail,having existed only briefly
(E) mail,but was existing only briefly
可以解释下C为什么不对吗?
existing动名词做主语，made作它的谓语不可以吗？也就是说However,existing only briefly
before the telegraph system made it obsolete.