In the constant$-$growth dividend valuation model, the required rate of return on common stock $($i$.$e$.,$ cost of equity capital$)$ can be shown to be equal to the sum of the dividend yield plus the $\_\_\_\_.$

a$.$ yield$-$to$-$maturity

b$.$ present value yield

c$.$ risk$-$free rate

d$.$ dividend growth rate

e$.$ none of the above