Which situation is an example of an observational study?

A. testing the effectiveness of a mouthwash by allowing 1 group to use it and comparing the results with those of a group that don;t use it.

B. Collecting the blood pressure reading of a group of elderly individuals in a small town

C. Having customers fill out a questionnaire about their favorite brand of toothpast

D. Diving a class into thirds, giving each third a different amount of time to read, and then testing coprehension