The average curve and the marginal curve are U-shaped in the short-run and flatter in the long-run due to________

Analyze the question: It asks why the average and marginal cost curves are U-shaped in the short run, but flatter in the long run.

Explain the concept:

• Short run: At least one factor of production (e.g. capital or land) is fixed. As a firm adds more variable inputs, output first rises more than proportionately, so cost per unit falls. Eventually the law of diminishing returns (variable proportions) sets in, so cost per unit rises. This gives the U-shape.
• Long run: All factors are variable, so the firm can change its scale of production. The law of diminishing returns no longer applies in the same way. Instead, returns to scale and economies/diseconomies of scale determine the shape of cost. Average cost falls as the firm enjoys economies of scale, stays roughly constant over a range, and rises only later when diseconomies of scale appear. This makes the long-run curve flatter and more gently U-shaped (or saucer-shaped).

Solution by elimination:

• B. Money cost: This is just the actual monetary expenditure on production. It does not explain curve shapes.
• C. Opportunity cost: This is the value of the next best alternative forgone. It is not related to the shape of the curves.
• D. Cost functions: These are the relationships between cost and output. The curves are drawn from them, so they are not the reason for the flatter shape.
• A. Economies of scale: This is the reason the long-run curves are flatter. Because all inputs can be adjusted, increases in scale lower the average cost per unit over a wide range of output.

Answer: The best option is A. Economies of scale.

Note: The question's wording is a bit loose. The short-run U-shape is strictly due to diminishing returns, while the flatter long-run shape is due to economies of scale. Among the options given, only A fits.

Exam tip: Remember it this way: Short run = diminishing returns; Long run = returns to scale (economies/diseconomies of scale). If you see "long-run cost curve" in a JAMB question, think "scale".