In this case, we need to identify the control group and experimental group. When experiments are done it is said that a control group or variable is needed, this means that a group or variable is left unaffected by the condition we are trying to test, in this case, is Red #2, this is done because we need to contrast the effect of the factor of interest versus a regular/normal conditions, otherwise, we couldn't be sure that the effects we are seeing are due to the experimental factor, there are other ways to test the effect of something but for that, a gradient is needed and that is another subject. In the present case, the group that received Red #2 is the experimental group, with this was intended to see if they developed cancer versus those that didn't receive it, to see if there were differences between groups.