Respuesta :
im pretty sure the answer would be C, control group. not 100% sure
Answer: C. a control group
Explanation:
A control group is the group which is used in an experiment. It is kept as a standard or benchmark for the comparison of the changes that occur in an experimental variables.
In the given experiment no control group is used. That is all the seven groups were exposed to different types of commercial there was no group which was not exposed any of the commercial. Hence, the validity of the experiment is doubtful.