Empirical Explorations of Freud's or Erikson's Theories Are Relatively Rare

Empirical explorations of Freud's or Erikson's theories are relatively rare, largely because both theories

A) deal with concepts that are extremely complex.
B) are so general that specific tests are very difficult.
C) are developmental in orientation.
D) have overlooked central features of human development.
E) are already thoroughly researched.