A firm's average cost decreases in the long-run because of

In the long run, a firm's average cost decreases primarily because of increasing returns to scale. This occurs when a firm's output increases by a proportion greater than the increase in inputs. In simpler terms, when a company gets bigger, it becomes more efficient at production. Consider the following reasons:

• **Economies of Scale**: As production scales up, certain costs such as fixed costs are spread over a larger number of goods, thus reducing the cost per unit.
• **Specialization and Division of Labor**: Larger firms often permit more specialization among workers and use more advanced technology, leading to increased productivity.
• **Bulk Purchasing**: Bigger firms can buy raw materials in large quantities at a discounted rate, which reduces the cost of production per unit.

Other options, like **diminishing average returns** or **decreasing marginal returns**, generally pertain to situations in the short run or under different conditions and would not directly explain a decrease in average costs in the long run. Additionally, while **decreasing average fixed cost** could reduce cost in the short run, in the long run, all costs are variable and focus instead shifts to how efficiently inputs are converted to outputs.