Answer choice D

Yes, D suggests that in the study, there was a placebo. However, with no results showing anything one way or another, the placebo effect seems to not exist in this case. There were no differences at all! E shows another way that there could have been differences between the high-BC population and the low-BC population. It could be, essentially, that the high-BC population also had other healthier habits that reduced their risks of cancer and heart disease, such as smoking less.

While I selected it through process of eliminaiton, I don't quite understand why D is correct from the explanation above. Could anyone explain it again please? Thank you!