Assume that interest rates on 20-year Treasury and corporate bonds with different ratings, all of which are noncallable, are as follows: Treasury-bond-7.72% A-rated - 9.649 AAA-rated-8.72% BBB-rated - 10.18% The differences in rates among these issues were most probably caused primarily by: O Real risk-free rate differences Tax effects Default risk differences O Maturity risk differences Inflation differences