Many drugs used in treating schizophrenia block the reception of dopamine by neurons. (Dopamine is a neurotransmitter, which, when released by the axons of one nerve, inhibits the firing of the next nerve.) This fact led to the idea that schizophrenia occurs when too much dopamine is produced. Suppose the following data on dopamine production were obtained. Schizophrenics Control Group
42 33
31 27
29 18

The study described above is an example of:

a. an independent-samples design;
b. a paired-samples design;
c. comparing a sample mean to a known population mean;
d. unknown; more information is required in order to decide.