Assume that interest rates on 20-year Treasury and corporate bonds are as follows: T-bond = 7.72% AAA = 8.72% A = 9.64% BBB = 10.18% The differences in these rates were probably caused primarily by:

a. Real risk-free rate differences.

b. Tax effects.

c. Inflation differences.

d. Default and liquidity risk differences.

e. Maturity risk differences.