Answer: 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.