The investigators concluded that the different types of movies had different effects on appetite. two variables in a study are said to be confounded if:

I believe the answer is: one cannot separate their effects on a response variable

The existence of confounding variables would ruin the whole experiment and make it produce useless results. To prevent confounding variables from happening, researches tend to separate the subjects into two groups. One group would be given the real treatment while the others would be given placebo.