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