Assume that interest rates on 20-year Treasury and corporate bonds with different ratings, all of which are noncallable, are as follows: T-bond -7.72%, A = 9.64%, AAA = 8.72% BBB = 10.18%. The differences in rates among these issues were most probably caused primarily by: Select one: a. Real risk-free rate differences. b. Default and liquidity risk differences. c. Maturity risk differences. d. Tax effects. e. Inflation differences