If participants who receive the thc (marijuana) pill are frequent marijuana users and participants who receive the placebo pill are first-time marijuana users, this is an example of a(n)

The correct answer is "confound".

Explanation:

In clinical trials, confounding is defined as a situation at which a known prognostic factor is unevenly distributed among the different groups of study. In this case, the prognostic factor of "frequency of marijuana use" is unevenly distributed between the group of participants receiving the treatment and the group of participants who receive the placebo pill. This is an example of confound, a situation that must be corrected in order to have valid results among the different groups of study.