This question uses the experimental study concept.
Doing this, we have that the walls analyzed are the individuals.
First, the concept of experimental study is presented:
In this question:
There is an intervention in the walls, they are painted, using three different paint compositions, and then they are exposed to sunlight, and after that the walls were analyzed, to study the degree of change in pigment of the paint, being compared between different paint compositions.
Thus, since the intervention was in the walls, they are the individuals in this experiment.
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