According to the capital structure trade-off model: O a. The optimal capital structure minimizes the company's weighted average cost of capital. O b. There is no optimal capital structure, but instead there is a hierarchy of sources of capital. O c. There is no optimal capital structure, and companies should aim to maximize debt financing, since they thereby minimize their tax payments. O d. The optimal capital structure minimizes the company's market value. O e. The optimal capital structure minimizes the company's cost of equity