Which of the following can be used as the before$-$tax cost of debt $(R_D)$ in the WACC calculation?

a$.$ Yield to Maturity $($YTM$)$ associated with the firm's bonds.

b$.$ Return on equity if the firm has no preferred stock.

c$.$ Average coupon rate of the firm's bonds.

d$.$ Dividend yield associated with the firm's common stock.