Which of the following best describes the law of diminishing marginal returns?

a. When more and more of a variable resource is added to a given amount of a fixed resource, the resulting change in output will eventually diminish and could become negative.
b. The notion that as a person consumes more and more of a good, such as 12 ounce cups of lemonade, the marginal utility from each additional cup will tend to decline.
c. The empirical fact that positive economic profits will tend to decline over time as new firms attracted by the extra-normal profit opportunity enter the market.
d. When more and more capital per labor is used in production, the marginal product of labor eventually declines and could become negative.

a