参考答案：C

解析：

A short-run decrease in output will cause a firm's average fixed costs to increase because its fixed costs are spread over a smaller number of units. In terms of cost curves, average fixed cost never slopes upward, so a decrease in output never reduces average fixed costs. The average variable cost, average total cost, and marginal cost curves all have upward sloping components along which a lower level of output would result in a lower cost.