Why Was the Loose Network of Antislavery Sympathizers Who Conveyed

Why was the loose network of antislavery sympathizers who conveyed runaway slaves north to freedom known as the Underground Railroad?

A) The route was primarily by subway in the major eastern cities.
B) Runaway slaves were directed by conductors from one "station" to the next.
C) Slaves traveled in secret compartments in Pullman sleeping cars, whose only access was from below the car.
D) Many of the segments north were through secret tunnels constructed to bypass major roads.