The U.S. economy expanded in the first quarter at the slowest pace in two years as American consumers reined in spending and companies tightened their belts in response to weak global financial conditions and a plunge in oil prices. Gross domestic product rose at a 0.5 percent annualized rate after a 1.4 percent fourth-quarter advance, Commerce Department data showed Thursday. The increase was less than the 0.7 percent median projection in a Bloomberg survey and marked the third straight disappointing start to a year. Shaky global markets and oil’s tumble resulted in the biggest business-investment slump in almost seven years, and household purchases climbed the least since early 2015, the data showed. While Federal Reserve officials on Wednesday acknowledged the softness, they also indicated strong hiring and income gains have the potential to reignite consumer spending and propel economic growth.