Assume That Interest Rates on 20-Year Treasury and Corporate Bonds

Assume that interest rates on 20-year Treasury and corporate bonds with different ratings, all of which are noncallable, are as follows:

The differences in rates among these issues were most probably caused primarily by:

A) Real risk-free rate differences.
B) Tax effects.
C) Default risk differences.
D) Maturity risk differences.
E) Inflation differences.