Which of the Following Is NOT True of Familial Aggregation

Which of the following is NOT true of familial aggregation of a trait?

A) One should compare the clustering of the trait in affected families compared to the general population
B) Family clustering proves there is a genetic contribution to the trait
C) Increased trait sharing in monozygotic compared to dizygotic twins suggests a genetic contribution to the trait
D) Traits with a genetic liability will show familial aggregation
E) These studies can be plagued by ascertainment bias