Why Is Freud's Theory of Personality Difficult to Disprove

Why is Freud's theory of personality difficult to disprove?

A) Freud's theory is so well constructed that it is difficult to find substantial flaws within it.
B) It is so widely accepted that few scientists are willing to attempt any research that would disprove the theory.
C) The research to disprove the theory would require such large investments of time and money that the research would be impractical.
D) The major concepts of Freud's theory are based on the unconscious and are therefore difficult to measure.