The expansion of which infrastructure caused the growth of the oil and steel industries?

Railroads

Explanation: Railroads made it significantly easier to get raw materials (such as oil and steel) across the country. If you want a specific example, look at the Trans-Continental Railroad

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this defines the ability for a flex item to grow if necessary. it accepts a unitless value that serves as a proportion. it dictates what amount of the available space inside the flex container the item should take up.

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