The Major Problem with Family Incidence Studies That Ask Whether

The major problem with family incidence studies that ask whether diseases "run in families" is that even when they find a higher prevalence of a certain illness in families with one ill proband

A) they cannot determine if a single gene or multiple genes are involved.
B) they cannot determine if dominant genes or recessive genes are involved.
C) the finding is consistent with both genetic and environmental causation.
D) they can only study families where there are identical twins.