I think is D i hope this helps
Answer:
Steel
Explanation:
During the second industrial revolution,which building because one of the most important in the world? <em>rail</em><em> </em><em>roads</em><em> </em><em>made</em><em> </em><em>resources</em><em> </em><em>and</em><em> </em><em>products</em><em> </em><em>easier</em><em> </em><em>to</em><em> </em><em>transport</em>
