Diminishing marginal returns are a result of increasing input after an optimal capacity has been reached.
what is returns to scale?
- In economics, returns to scale are the quantitative changes in output of a firm or industry caused by a proportionate increase in all inputs.
-  The production process is said to display growing returns to scale if the quantity of output increases by a bigger proportion—for example, if output increases by 2.5 times in response to a doubling of all inputs. 
- Such economies of scale may develop as a result of increased efficiency as the firm expands from small- to large-scale operations. 
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