The amount of a substance excreted from the kidneys is calculated by the equation: filtration + secretion - reabsorption.
So, the correct answer is C.
This equation represents the processes that occur in the kidneys to maintain proper balance of substances in the body.
Filtration removes waste and excess substances from the blood, while secretion adds specific unwanted substances to the filtrate. Reabsorption returns necessary substances back to the blood.
The net result of these processes determines the amount of a substance that is ultimately excreted through urine.
Hence, the answer of the question is C.
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