Read the following passage, then answer the question.
---
PASSAGE L1 — The Echo of Iron
For three generations, the embers of Hopewell Furnace had glowed red against the hills of Berks County. Fourteen-year-old Thomas stood near the roaring blast furnace, his cheeks flushed with heat. His job was to sort limestone and charcoal, feeding the furnace that transformed raw Pennsylvania ore into sturdy pig iron. His father, the head founder, watched the molten stream spill into the sand molds with an expression of quiet pride. "Iron isn't just metal, Thomas," his father said above the roar of the water wheel. "It's the skeleton of every bridge, rail, and plow that builds this country."
Yet Thomas often gazed past the soot-stained wooden stacks toward the forest ridge. While his father saw purpose in the smoke and sparks, Thomas wondered about the world beyond the village — the bustling ports of Philadelphia and the western frontier expanding along canal waterways. Carrying a heavy wooden hod of limestone, he paused at the edge of the casting house. He respected his father's dedication, but he knew in his heart that his own future lay not in shaping molten metal, but in designing the engines and structures that metal would power.
---
What does the father mean when he describes iron as "the skeleton of every bridge, rail, and plow"?
- AIron is a fragile material that breaks easily under pressure.
- BIron forms the hidden, essential structural framework that supports modern progress.
- CIron is dangerous and resembles human bones when cooled in sand molds.
- DIron can only be used after animals have cleared the land for development.