Assume that interest rates on 15-year noncallable Treasury and corporate bonds with different ratings are as follows: • T-bond = 7.72% • AAA = 8.72% • A = 9.64% • BBB = 10.18% The difference in rates among these issues were most probably caused by: a. Tax effects. b. Default risk differences. c. Maturity risk differences. d. Inflation differences.