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Question 8 of 10 Which situation is an example of an observational study? X A. Having customers fill out a questionnaire about their favorite brand of toothpaste B. Dividing a class into thirds, giving each third a different amount of time to read, and then testing comprehension C. 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 VO D. Collecting the blood pressure readings of a group of elderly individuals in a small town X Incorrect PREVIOUS $18​