Which of the following statements about bonds is true?

a) A call provision represents an obligation to the holder to purchase back the bond at some point in the future.

b) A bond with a sinking fund means that the bondholder will receive a portion of their principal back each time a payment to the fund is made.

c) A bond with an AA rating will have a higher risk premium associated with it compared to a bond with a B rating.

d) A bond may trade at a premium or a discount, depending on what the current interest rate is.