Which of the following statements is correct?

a. The constant-growth dividend model assumes that dividends will grow at a constant rate forever.

b. All the answers are correct.

c. Legally, common stockholders have unlimited liability; that is, their losses are not limited to the original amount of their investment in the firm, and their personal assets can be taken to satisfy the obligations of the corporation.

d. Whenever the required rate of return exceeds the dividend growth rate, the constant-growth model provides invalid solutions.

e. Preferred stock is not legally a form of equity and preferred stock dividends are paid by the issuer with before-tax dollars.