GWD语法题，求解！

kobesoros

The greatest road system built in the Americas prior to the arrival of Christopher Columbus was the Incan highway, which, over 2,500 miles long and extending from northern Ecuador through Peru to Southern Chile.

a。Columbus was the Incan highway, which, over 2,500 miles long and extending
b。Columbus was the Incan highway, over 2,500 miles in length, and extended
c。Columbus, the Incan highway, which was over 2,500 miles in length and extended
d。Columbus, the Incan highway, being over 2,500 miles in length, was extended
e。Columbus, the Incan highway was over 2,500 miles long, extending

答案是E。我选对了。可是有一个问题在我脑海里出现。我们可以看出E是同位语，但是，曾经有人说过逗号不能连接两个句子。E不就是逗号连接两个句子吗？为什么不能算错呢？以后如何区分？谢谢！

sdcar2010

There is only one sentence.  The greatest road system, the Incan highway was . . .