Concept:The shape of cost curves changes between the short run and the long run because of changes in the firm's ability to adjust its scale of production.
Explanation:In the short run, at least one factor of production is fixed, so the law of diminishing returns makes average cost and marginal cost U-shaped.
In the long run, the firm can vary all inputs and expand its plant size.
As the firm grows, economies of scale help reduce average cost and marginal cost.
This makes the cost curves flatter in the long run than in the short run.
Therefore, the flattening of the average and marginal curves in the long run is due to economies of scale, also known as returns to scale.
Answer:A. Economies of scale