"…but the completion of this mighty work, which connects the two oceans, is a still greater blessing. in the olden times, when camels – those ships of the desert – were the means used for transportation, for the furtherance if commercial traffic, it was found that wherever the caravans stopped there would spring up cities. and there would be evidences of civilization. so with this great work. it will populate the vast territory and be the great highway of the nations; their merchants will cross it to trade with us."

- excerpt from “east and west: completion of the great line spanning the continent.” the new york times, may 11, 1869.

which transportation network in the united states that was completed in 1869 is being referenced in the passage?
a) erie canal
b) panama canal
c) national road
d) transcontinental railroad