Read the excerpt from The Building of Manhattan. A temporary, small narrow-gauge track system was installed on each floor as it was needed. This enabled the material to be moved from the truck at ground level onto dump carts, raised by elevator to the designated floor, wheeled onto the track, and moved quickly to the exact spot needed. Turntables built into the track allowed the carts to be shifted about in any direction.

Which illustration would best give the reader a clearer understanding of the text?

an illustration of a truck carrying building materials
an illustration of materials being moved onto building floors
an illustration of the partially constructed building
an illustration of a riveter bonding a pair of steel beams