Federal Reserve Chair Janet Yellen Semiannual Monetary Policy Report To The Senate Banking Committee

Janet Yellen, chair of the U.S. Federal Reserve, listens during a Senate Banking Committee hearing in Washington, D.C., U.S., on Thursday, Feb. 11, 2016. Yellen said the Fed was taking another look at negative interest rates as a potential policy tool if the U.S. economy faltered, after central banks in Europe were able to drive borrowing costs below zero. Photographer: Andrew Harrer/Bloomberg via Getty Images