Serendipity Inc. is re-evaluating its debt level. Its current capital structure consists of 80% debt and 20% common equity, its beta is 1.60, and its tax rate is 35%. However, the CFO thinks the company has too much debt, and he is considering moving to a capital structure with 40% debt and 60% equity. The risk-free rate is 5.0% and the market risk premium is 6.0%. By how much would the capital structure shift change the firm's cost of equity?

A: -5.2% B: -5.78% C: -6.36% D: -6.99% E: -7.69%