Final answer:
The primary difference between control and experimental groups in an experiment is the exposure to the independent variable, where the experimental group encounters it and the control group does not.
Step-by-step explanation:
The primary difference between the control and experimental groups in an experiment is A) the independent variable. The experimental group is exposed to the experimental manipulation, meaning they will experience the independent variable's influence. Meanwhile, the control group is not exposed to the experimental manipulation so any changes in the control group can be attributed to other factors than the independent variable. This setup ensures that any observed differences in the dependent variable between the two groups can be attributed to the independent variable. Placebos, when given to control groups, help mitigate the placebo effect and keep the study results as unbiased as possible.