People who walked for half an hour three times a week and people who sat quietly reading a book for half an hour three times a week participated in a one-year study. At the end of the year, the change in each participant's blood pressure was measured. The changes were compared for the two groups. This is an observational study rather than a randomized experiment because A. a random sample of participants was used
B. the two groups were compared at the end of the study.
c. blood pressure was measured at the beginning and end of the study
d. the participants were not randomly assigned to either walk or read.