Answer:
The correct answer for the execise is that teachers have diferent amount of teaching experience and that is a potential confound in the study.
Step-by-step explanation:
First of all, to conduct a comparison study to a certain dependent variable the researcher has to control all the conditions and only use the dependant condition as the different factor. However in our case, we have one major problem, the teachers should follow the same strategies, same classes, and practically do the same except the book employment. Because the teacher experience can affect the data, the study, the population, and the results. This research is potentially destined to be biased.