Read the passage from anthem, by ayn rand. in this passage, the narrator is exploring an old, abandoned underground tunnel. but we pulled at the grill [on the ground] and it gave way. and then we saw iron rings as steps leading down a shaft into a darkness without bottom. it was a great tunnel. its walls were hard and smooth to the touch; it felt like stone, but it was not stone. on the ground there were long thin tracks of iron; it felt smooth and cold as glass. we knelt, and we crawled forward, our hand groping along the iron line to see where it would lead. but there was an unbroken night ahead. what is the most likely explanation for the walls described in the bold text? a. the walls are composed of natural rock. b. the walls are man-made, of concrete. c. the walls felt like sandpaper. d. the walls are wet.