Which of the following is an example of the law of diminishing marginal returns?
A. Holding capital constant, when the amount of labor increases from 5 to 6, output increases from 20 to 25. Then when labor increases from 6 to 7, output increases from 25 to 28.
B. When labor increases by 20% and capital decreases by 15%, output remains the same.
C. Holding capital constant, when the amount of labor increases from 7 to 8, output increases from 36 to 42. Then when labor increases from 8 to 9, output increases from 42 to 50.
D. When capital and labor both increase by 20%, output increases by only 15%.