B. A treatment's effects spreading from the experimental group to the control group.
<h3>Why would a control condition be used in an experiment?</h3>
When conducting an experiment, a control is a factor that doesn't change or is unaffected by other factors. It acts as a benchmark or point of comparison against which other test results are assessed.
The spread of the treatment from the experimental group to the control group is referred to as diffusion, and it has the effect of minimizing the difference between the two situations. When a group that has not previously received therapy hears about the care the other group is receiving, it expects the same or comparable care.
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