A study involving middle-school students investigates the link between a diet high in processed foods and rates of depression. Researchers collected the urine of the participants and measured levels of sodium and potassium. A diet high in processed food, such as fast food, is high in sodium and low in potassium. The students were also evaluated on a 100-point scale for depressive symptoms. The study found that high levels of sodium and low levels of potassium were both associated with higher depression levels. 

(a) Is this an experiment or an observational study? 

(b) Can we conclude that eating a diet high in sodium and low in potassium increases the likelihood of having depression? Why or why not? 

(c) Give a possible confounding variable in this study, and explain how it meets the definition of a confounding variable.