The control group might be defined as the one that does not receive any treatment -meaning the independent variable-. So in this example, the control group are the dogs that receive water (option B). These dogs do not get caffein, which is the independent variable.
During an experiment the researcher compares at least two groups: a control group and an experimental group. Both groups are composed of individuals coming from the same population, so they are identical in all aspects except for the independent variables.
So, in the exposed example, the population is composed of dogs. From this population, the researcher selects one control group and two experimental groups. The independent variable in this study is the amount of caffeine provided to dogs.
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