4. The following table summarizes the yields to maturity on several one-yea r, zero- coupon securities: Security Treasury AAA corporate BBB corporate B corporate Yield (%) 3.1 3.2 4.2 4.9 What is the price (expressed as a percentage of the face value) of a one-year, a. zero-coupon corporate bond with a AAA rating? b. What is the credit spread on AAA-rated corporate bonds? c. What is the credit spread on B-rated corporate bonds? d. How does the credit spread change with the bond rating? Why