Based on historical data, which of the following tended to be most unstable over time?

Which of the following statements is true?

A) In the long run, a firm cannot vary any of its inputs. B) In the long run, a firm can vary all its inputs. C) In the short run, a firm cannot vary any of its inputs. D) In the short run, a firm can vary all its inputs.

In the above figure, the line represented by the "4" is the

A) average fixed cost. B) marginal revenue. C) average total cost. D) marginal cost.