Aresearcher wanted to compare the relative effectiveness of two techniques for reducing anxiety: a drug and a relaxation exercise. the researcher gave the experimental group both the drug and the relaxation exercise. the control group was given neither the drug nor the exercise. the difference between the drug and the relaxation exercise cannot be compared in this experiment because the experiment involves a

Confounding

Explanation: Confounding is the term that defines the influence occurring in dependent and non-dependent components due to the external/outside factor. This outside factor can cause the effect on overall experiment and its design.

According to the experiment executed in the question, experimental group and control group cannot be compared because the confounding factor are present in the experimental group so that must have influence their  characteristics. Thus, the outcome of the experiment while comparing will be impacted.