One Complication in Determining Whether the Traits of Extraversion and Neuroticism

One complication in determining whether the traits of extraversion and neuroticism explain (i.e. cause) differences in affectivity and happiness is that

A) measures of extraversion, neuroticism, affectivity. and happiness overlap and therefore may not be assessing independent factors.
B) researchers have been unable to tease out genetic from environmental causes.
C) extraversion and neuroticism may represent a single rather than two independent dimensions of personality.
D) all of the above