WASHINGTON (AP) — Federal Reserve Chair Janet Yellen on Friday defended the central bank's extraordinary efforts to fight the Great Recession and said they might be needed again. During the recession, the Fed pushed short-term interest rates to zero. When the economy needed more help, it took the extraordinary step of buying hundreds of billions of dollars' worth of bonds to push long-term interest rates lower. Now the economy is improving: It grew at a sprightly 3.1 percent annual pace from April through September, and the unemployment rate has tumbled to a 16-year low 4.2 percent. "The U.S.