Fifty dog owners and 50 people who do not own dogs are each given a test to evaluate their stress levels. Dog owners are found to have significantly lower stress levels than people who do not own dogs. Which of the following statements is true?
I. This study provides good evidence that owning a dog is effective in reducing stress.
II. It is likely that a confounding variable could be a contributing explanation for the difference between these two groups.
III. This is an observational study, not an experiment.
A. I only
B. II only
C. III only
D. II and III only
E. I and III only