Option D: It helped him predict that the demand for steel to build railroad tracks and bridges would grow.
Step-by-step explanation: Andrew Carnegie a Scotland born American industrialist and a philanthropist became a significant contributor in the expansion of the American steel industry in the late 19th century. As a young boy, he started working in a Pittsburgh based cotton industry. Then in 1859, his career flourished when he joined to the position of Division Supervisor in Pennsylvania Railroad. His work experience with Pennsylvania Railroad helped him estimate the growing demand for steel to build railroad tracks and bridges.