The correct answer is A) There was a massive increase in foreign trade.
After World War II, the United States had a strong, growing economy because there was a massive increase in foreign trade.
When hostilities of World War II were over and the winning allied nations finished the negotiations of peace, the United States resulted in favored in that they not only had physically supported the European Allies with soliders, supplies, and weaponry but also borrowed money for the war effort. Then as part of the Marshall Plan to reconstruct some countries of Europe, the US massively increased investments and foreign trade, producing great earnings and economic growth for the US.