Chronic Fatigue Syndrome (CFS) Is a Debilitating Disease of Unknown

Chronic fatigue syndrome (CFS) is a debilitating disease of unknown etiology that is estimated to affect 17 million people worldwide. Studying peripheral blood mononuclear cells from patients with CFS, researchers identified DNA from a human gamma-retrovirus (XMRV) in 68 of 101 patients, compared with 8 of 218 healthy controls.
What may we conclude from these data?

A) XMRV DNA appears to be much more common among patients with CFS than among healthy controls.
B) XMRV DNA appears to be much more common among healthy controls than among patients with CFS.
C) XMRV DNA is approximately as frequent among patients with CFS as among healthy controls.
D) We cannot state any conclusion, because the data came from an observational study.