Which one of the following statements related to taxes is correct? Select one: a. The marginal tax rate must be equal to or lower than the average tax rate for a firm. b. The tax for a firm is computed by multiplying the firm's current marginal tax rate times the taxable income. c. Additional income is taxed at a firm's average tax rate. d. Given the corporate tax structure in 2012, the highest marginal tax rate is equal to the highest average tax rate. e. The marginal tax rate for a firm can be either higher than or the same as the average tax rate.