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% AAA = 8.72% A = 9.64% BBB = 10.18% The differences in rates among these issues were most probably caused primarily by: a. Real risk-free rate differences. O b. Tax effects. O c. Default risk and liquidity differences. O d. Maturity risk differences. O e. Inflation differences