A firm sells one product with a contribution margin per unit of $20 and a contribution margin ratio of .4. Which of the following is TRUE? a. For every unit sold, $8 must go toward covering variable costs. O b. For every $1 of sales revenue, $.40 is available to cover fixed costs and generate operating income, c. For every unit sold, $20 must go toward covering variable costs. O d. For every $1 of sales revenue, $.60 is available to cover fixed costs and generate operating income