A teacher is interested in knowing if riding a bus to school results in students having more friends at school. She asks 80 students who ride a bus and 60 students who get a ride to school by their parents how many good friends they have at school. She found that students who ride a bus have on average 5 good friends while students who ride with their parents have 4 good friends at school. Is this an example of a controlled experiment or observational study?
a.Observational study
b.Controlled experiment
c.Neither
d.Both