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% A = 9.64% AAA = 8.72% BBB = 10.18% The differences in rates among these issues were most probably caused primarily by:

The question is missing the options which are below:

A Real risk-free rate differences.  

B Tax effects.  

C Default risk differences.  

D Maturity risk differences.  

E Inflation differences.  

The correct answer is option C,default risk differences.

Explanation:

Default risk is the increase in return given to an investor to compensate the investor for the likely losses that may arise due to the inability of the borrower to make funds available to the investor on the maturity date or even in required amount.

Different debt instruments have different default risk depending on their credit rating as rated by international rating agencies.Such rating is a function of many factors,which includes:

Balance sheet position

Profitability

Liquidity strength of the company

Macro-economic factors and some others.

Liquidity refers to the ability of the company to settle obligations such as repayment of bonds and interest  when due.

Invariably,liquidity has a higher impact in determining credit rating as well as default risk of an instrument.

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