Answer:
C. fall; increasing marginal costs.
Explanation:
Option C is the correct answer because initially, the average costs fall due to increasing return or production of more units. When output increases, the average fixed cost slopes downwards. Moreover, when the average cost falls, marginal cost also falls and it starts rising as the marginal cost cuts the average cost at its minimum point. However, after cutting at the minimum point, marginal cost increases, and due to which average cost also increases.