Interviews with senior Fed officials and advisors suggest they remain confident the U.S. economy will be ready for a modest policy tightening in the June-September period, while any subsequent rate hikes will probably be slow and depend on how markets will behave. While they are hard-pressed to explain why bond yields have fallen so low, their confidence in the recovery stems in part from in-house analysis that shows falling oil prices are clearly positive for the U.S. economy.