Information gathered by a scientist abut the toxicity of chemical x and chemical y showed that they had individual safe limits for fish at particular concentrations. but when they were used together at the safe concentrations, there were extensive fish kills. this is an example

The correct answer is synergism.

Explanation:

In synergism, two or more substances interact with each other and produce a greater combined effect than the sum of the effect produced by them separately. So in synergism two chemicals enhance their overall effect when works together. For example, when a cancer patient is given chemotherapy and radiation together it shows better results than when these two therapy are used separately.

So, in this case, X and Y chemicals individually have a limited effect on fish but when used together their combined effect becomes greater which becomes toxic to the fishes that show that it is an example of synergism.

i think your answer is c. measurement system in which the medication dosage was calculated.