High$-$yield bonds offer higher yields over the same time period relative to higher$-$quality bonds because $\_\_\_\_\_\_\_.$

a$.$ the issuing entity knows bond markets are competitive, so they raised their coupon payments to compete for investors' cash

b$.$ the issuing entity is willing to pay more because it typically has a major acquisition on the horizon that will be even more profitable for the entity

c$.$ the issuing entity has higher cash flows than other issuers, so they can afford the higher payments

d$.$ the issuing entity has lower credit quality relative to other issuers