Which situation is an example of an observational study? a.) finding out tye number of monthly burglaries of a residential neighborhood for a statistical study for the police b.) testing the effectiveness of a new hair product by allowing one group to use it and comparing results against a group that doesn't use it. c.) calling a sample of registered voters before an election to find out what issues concern them most d.) collecting the cholesterol readings of a group of people in a certain age group in a small town