Do Doctors in Managed Care Plans Give Less Charity Care

Do doctors in managed care plans give less charity care? Researchers chose 60 communities at random, then chose doctors at random in each community. In all, they interviewed 10,881 doctors. Overall, 77.3% of the doctors said they had given some care free or at reduced rates because of the patient's financial need in the month before the interview. Doctors who received at least 85% of their practice income from managed care plans were significantly less likely than other doctors to provide charity care.
This study is a(n)

A) randomized comparative experiment.
B) experiment, but there is no control group.
C) census.
D) sample survey.