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

a$.$ The rating agencies change the bond's rating from Baa to Aaa.

b$.$ Making the bond a first mortgage bond rather than a debenture.

c$.$ Adding a sinking fund.

d$.$ Adding a call provision.

e$.$ Adding additional restrictive covenants that limit management's actions.