Answer:
vary along with one another.
Step-by-step explanation:
Confounding variable is a term in research methodology that describes a form of an additional variable that the researcher did not prepare for.
In other words, it is considered as an external influence that alters the effect of a dependent and independent variable. For example, the weather could be considered as a confounding variable that may change the effect of the experiment design.
However, because confounding variables vary along with one another, they have the tendency to ruin an experiment and lead to worthless results. They can also initiate bias.
Hence, the primary reason that confounding variables pose problems for researchers is that they tend to VARY ALONG WITH ONE ANOTHER.