The correct answer is the skills of the children in math.
In statistics, a confounding variable also known as a confounding factor, a confusing variable or confounding factor is a strange variable that is positively or negatively associated with both the explanatory variable and the response variable. We need to control these factors to avoid incorrect beliefs that the response variable is associated with the explanatory variable.
Confusion is a major threat to the validity of inferences made about statistical associations. In the case of a confounding variable, the association observed with the response variable should be attributed to the confounding factor, instead of the explanatory variable. In science, we test for confounding factors including these third, or fourth, or fifth, or sixth variables, in statistical models to explain the association of interest.