Determine the Evaluation Guideline That Applies Best in Questioning the Results

Determine the evaluation guideline that applies best in questioning the results of the described study.
-A new publisher of educational texts reported: "We had teachers use our texts in six Algebra I classes and other
Teachers use another leading text in six Algebra I classes. We tested the students at the end of the semester and
Students using our text scored 11 points higher."

A) Consider possible confounding variables.
B) Consider the source.
C) Consider the sampling method.
D) Consider the population.