Under Normal Conditions, Which of the Following Would Be Most

Under normal conditions, which of the following would be most likely to increase the coupon rate required to enable a bond to be issued at par?

A) adding a call provision.
B) the rating agencies change the bond's rating from baa to aaa.
C) making the bond a first mortgage bond rather than a debenture.
D) adding a sinking fund.
E) adding additional restrictive covenants that limit management's actions.