Assumptions of the Sustainable Growth Rate model allow that D) the amount of new debt that can be added to a firm's capital structure is equal to the amount of net income 2) the amount of new debt that can be added is in proportion to the firms current debt-to-assets ratio 3) the amount of new debt that can be added to a firm's capital structure is equal to the amount of retained earnings 4) the amount of new debt that can be added is in proportion to the firm's current debt-to-equity ratio 5) the amount of new debt that can be added is in proportion to the firm's current debt-to-retained earnings