Why Is Direct Behavioral Observation by Therapists Inefficient

Why is direct behavioral observation by therapists inefficient?

A) Many clients don't want their therapist intruding on their personal lives.
B) It is expensive and inefficient for therapists to try to conduct formal observations outside the office.
C) Being present takes the therapist from outside observer to participant, which may influence the behavior being observed.
D) All of the above.
E) Direct behavioral observations are not inefficient.