Please answer the question.

Exhibit 11-4 Units of Output Variable Cost Total Cost Marginal Cost (dollars) (dollars) (dollars) 0 0 80 60 140 60 100 180 40 120 200 20 + 160 240 40 210 290 50 300 380 90 Refer to Exhibit 11-4. How much are average fixed costs (in dollars) at 4 units of output? O 20 40 60 840Average variable cost: first tends to decrease, and then increase as output expands. remains unchanged as output expands. O always increases as output increases. O always decreases as output expands