语法题请教20071203

问下题目

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.

1. Columbus was the Incan highway, which, over 2,500 miles long and extending
2. Columbus was the Incan highway, over 2,500 miles in length, and extended
3. Columbus, the Incan highway, which was over 2,500 miles in length and extended
4. Columbus, the Incan highway, being over 2,500 miles in length, was extended
5. Columbus, the Incan highway was over 2,500 miles long, extending

答案是E,我选A, E中短语和主句之间只有一个",",可以吗?

我认为E应该可以理解成同位语，主语是the Incan highway 这样前面做同位语修饰主语，只用一个逗号是没问题的。