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%6AAA=8.72%A=9.64%BBB=10.18% The differences in rates among these issues were most probably caused primarily by: a. Moturity risk differences. b. Tax effects. c. Default risk and liquidity differences. d. Real risk-free rate differences e. Inflation differences