What Is a Major Shortcoming in the Field of Behavioral

What is a major shortcoming in the field of behavioral genetics in terms of their studies on human personality traits?

A) Behavioral geneticists are unable to conduct controlled research studies on human subjects.
B) Behavioral geneticists are unable to scientifically validate anything.
C) Behavioral geneticists are unable to conduct studies on animals, only on humans.
D) Behavioral geneticists conduct their studies by looking at single individuals over a long period of time, thus slowing the rate at which they can gather data.