NEW YORK ( TheStreet) -- SunTrust ( STI) led major U.S. banks higher on Thursday, with shares rising 1.6% to close at $35.98. The broad indices all ended higher, after Janet Yellen answered questions before the Senate Banking Committee, which considered her nomination by President Obama to succeed Ben Bernanke as the next chairperson of the Federal Reserve. Yellen currently serves as the central bank's vice chair. Yellen has been a very strong supporter of the Federal Reserve's "highly accommodative" stimulus policy, which has included keeping the short-term federal funds rate in a range of zero to 0.25% since late 2008. That's the Fed's main policy tool, but through its "QE3" stimulus policy, the central bank has been making monthly net purchases of $85 billion in long-term securities, in an effort to hold down long-term interest rates. The QE3 purchases have continued since September 2012. The stock market has ramped up over the past two years, fed in part by the historically low interest rate environment. Addressing the eventual rise in short-term rates and tapering of the Fed's bond purchases, Yellen said "It's important not to remove support, especially when the recovery is fragile, and the tools available to monetary policy should the economy falter are limited, given that short-term interest rates are at zero . . . I believe it could be costly to withdraw accommodation." After Senator Mike Crapo (R., Idaho) -- the ranking Republican member of the Senate Banking Committee -- expressed concern about the ballooning of the Fed's balance sheet, with a limited boost to U.S. GDP growth and "very serious risks" from quantitative easing, Yellen said "these purchases have made a meaningful contribution to economic growth and to improving the outlook." "The purpose of these purchases was to push down longer-term interest rates. We have seen interest rates fall very substantially. Lower mortgage rates particularly have been . . . a positive factor in generating a recovery in the housing sector." When Crapo asked how long "such extreme levels" of quantitative easing could continue, Yellen said "I would agree that this program cannot continue forever, that there are costs and risks associated with the program."