The increased use of automation and less use of the work force in companies has caused a trend towards an increase in O a. variable costs and no change in fixed costs. Ob. variable costs and a decrease in fixed costs. O c. both variable and fixed costs. O d. fixed costs and a decrease in variable costs. Contribution margin . equals sales revenue minus variable costs. O b. is calculated by subtracting total manufacturing costs per unit from sales revenue per unit. O c. is always the same as gross profit margin. O d. excludes variable selling costs from its calculation