Which of the Following Statements Is True

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 which vary with the output level.
D) When marginal productivity of a variable input is falling then marginal costs of production must be rising.
E) When marginal cost is below average cost, average cost rises; when marginal cost is above average cost, average cost falls.