Gogo Bus Company produces model toy car. Each model costs $40 of variable costs to make. 2,800 models were sold this month at $200 each. Fixed costs per unit for this month were $4. How much does Gogo Bus's operating income change for each $1,000 increase in revenue per month? Select one: O a. Not enough information to determine the answer. O b. $ 800 O c. $ 500 d. $ 700