Which of the following statements is true?

a. The law of diminishing returns states that beyond some point, the marginal product of a variable resource continues to rise.

b. The marginal product is the change in total output by adding one additional unit of a fixed input.

c. Fixed costs are costs that vary with the output level.

d. If marginal productivity of a variable input falls, then marginal costs of production must rise.

e. When marginal cost is below average cost, average cost rises; when marginal cost is above average cost, average cost falls.