the greatest road system

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 // the non -essential clause after ,which lacks any verb. Incorrect.

B. Columbus was the Incan highway, over 2,500 miles in length, and extended // Correct

C. Columbus, the Incan highway, which was over 2,500 miles in length and extended // It adds extra relative pronoun which between the subject 'railroad system'and the verb 'was'. The extra which is not needed. Eliminated.

D. Columbus, the Incan highway, being over 2,500 miles in length, was extended // Changes the meaning as if the railroad system was expanded after it was built. Eliminated.

E. Columbus, the Incan highway was over 2,500 miles long, extending // Could be correct, though i am not very positive because , the Incan highway was over 2,500 miles long, is merely a non essential modifier and if you neglect the stuff between commas the sentence is :

The greatest road system built in the Americas prior to the arrival of Christopher Columbus was the Incan highway extending from northern Ecuador through Peru to Southern Chile.

Reading the sentence it feels like the railroad system itself was extending rather than somebody extending it.

Arer B and D grammatically incorrect? and why?

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