Assume that interest rates on 20-year Treasury and corporate bonds are as follows:

T-bond	= 7.72%	A	= 9.64%
AAA	= 8.72%	BBB	=10.18%

The differences in rates among these issues were caused primarily by tax effects?

a) Default risk differences

b) Maturity risk differences

c) Inflation differences

d) Real risk-free rate difference