Which situation is an example of an observational study? O A. Testing the effectiveness of a mouthwash by allowing one group
to use it and comparing the results with those of a group that
doesn't use it
O B. Dividing a class into thirds, giving each third a different amount of
time to read, and then testing comprehension
O C. Collecting the blood pressure readings of a group of elderly
individuals in a small town
O D. Having customers fill out a questionnaire about their favorite
brand of toothpaste