Indicate whether each of the following statements is true or false. Explain why.
a. When the law of diminishing returns takes effect, a firm's average product will start to decrease.
b. Decreasing returns to scale occurs when a firm has to increase all its inputs at an increasing rate to maintain a constant rate of increase in its output.
c. A linear short-run production function implies that the law of diminishing returns does not take effect over the range of output being considered.
d. Stage I of the production process ends at the point where the law of diminishing returns occurs.