Iron and steel--by the late 19th century the US became the world leader in iron ore and steel exports.
Access to natural resources and a lot of labor, the US was able to access iron ore which was a hot commodity and necessary for creating steel. If countries had the ability to create steel then they would just buy the iron ore. If they could not produce steel, then US Steel was the export for them. Steel industries dominated states like Pennsylvania, Ohio, Michigan, and Indiana.
"<span>Membership in the League could draw the United States into future wars" would be the best option from the list, since the United States was heavily isolationist at this point, especially after fighting in World War I. </span>
