To determine the health benefits of walking, researchers conduct a study in which they compare the cholesterol levels of women who walk at least 10 miles per week to those of women who do not exercise at all. The study finds that the average cholesterol level for the walkers is 198, and that the level for those who don't exercise is 223. Which of the following statements is true?
I. This study provides good evidence that walking is effective in controlling cholesterol.
II. This is an observational study, not an experiment.
III. Although the study was conducted only on women, we can confidently generalize the results to men in the same age group.
A. I only
B. II only
C. III only
D. II and III only
E. I and III only