A company has outstanding debt with a market value of​ $250M and common stock with a market value of​ $550M. If its debt has a​ before-tax cost of​ 7%, a​ before-tax cost of equity of​ 10% and a corporate tax rate of​ 40%, what is its​ WACC? A. ​8.19% B. ​6.55% C. ​7.86% D. ​5.44%