Which situation is an example of an observational study?

OA. Calling a sample of registered voters before an election to find out
what issues concern them most

OB. 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. Interviewing college students to find what percentage expect a job
immediately after graduation

OD. Collecting the cholesterol readings of a group of elderly people in
a small town