During the period 1990 to 2017, the average yield on 3-month U.S. Treasury bills was 2.78%, the average inflation rate was 2.28%, the average yield on 30-year Treasury bonds was 5.22%, and the average return on 30-year Aaa-rated corporate bonds was 6.08%. The real risk-free short-term interest rate is ___

a 0.30%

b 0.50%

c 2.97%

d 4.76%