Northern Trust Corporation's common stock currently trades at $28 a share. It is expected to pay an annual dividend of $2.50 a share at the end of the year (D1 $2.50), and the constant growth rate is 4% a year. What is the company's cost of common equity if all of its equity comes from retained earnings? o 40% 15.0% o 8.9% None o 12.9%