Millions of retirees depend on Social Security to pay the bills, so much so that 62% of seniors count on it to provide half of their income or more. Unfortunately, nearly half of the U.S. adult population is missing key knowledge needed to make the most of those benefits.In recent quiz rolled out by Mass Mutual, a frightening 47% of Americans could not answer basic questions about full retirement age and spousal benefits -- two key components of Social Security.