Which of the following statements is CORRECT?

a$.$ If a firm finds that the cost of debt is less than the cost of equity, increasing its debt ratio must reduce its WACC.

b$.$ The capital structure that minimizes the firm's weighted average cost of capital is also the capital structure that maximizes its earnings per share.

c$.$ The capital structure that minimizes a firm's weighted average cost of capital is also the capital structure that maximizes its stock price.

d$.$ Other things held constant, if corporate tax rates declined, then the Modigliani$-$Miller tax$-$adjusted tradeoff theory would suggest that firms should increase their use of debt.

e$.$ A firm can use retained earnings without paying a flotation cost. Therefore, while the cost of retained earnings is not zero, its cost is generally lower than the after$-$tax cost of debt. dont tell me the answer just tell me what it mean and the logic behind it