Which of the following statements is correct regarding Social Security?

a.) Retirement benefits under socal security are intended to provide at least a 50% wage replacement ratio for everyone except for the very highest earners

b.) Non-working spouses of retirees receiving social security benefits are entitled to a 50% benefit regardless of the non-working spouse's age

c.) More than 20 percent of retirees rely on social security for more than 90% of their retirement income

d.) all employees in the United States are covered by social security and are required to contribute FICA taxes