One Way Researchers Try to Disentangle the Relationship Between Personality

One way researchers try to disentangle the relationship between personality and health is by conducting carefully controlled prospective studies. The important aspect of this research approach is that:

A) an initially healthy group of subjects is assessed on variables thought to be risk factors and is subsequently tracked over time.
B) neither the experimenters nor the subjects know the nature of the research.
C) it provides correlational data that indicate how the two variables are related without ruling out the possibility of some third, unidentified factor.
D) the subjects but not the experimenters know the nature of the research.