The law of diminishing marginal returns is caused by

A. poor quality fixed inputs.
B. insufficient amounts of the variable input.
C. some workers performing poorly on the job.
D. the existence of a fixed input that must be combined with increasing amounts of the variable input.

D. the existence of a fixed input that must be combined with increasing amounts of the variable input

Explanation:

The law of diminishing returns states that, at some point, increases in a factor of production will begin to cause smaller increases in output or diminishing output. For example, a company employs workers for manufacturing purposes in a factory and at some point production is at optimal level but at some point holding other inputs constant while increasing the other factor, production will begin to deviate from optimal and production will begin to decline

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