The Advantage of Using Outside Independent Observers or Pre/post Test

The advantage of using outside independent observers or pre/post test comparisons to evaluate a client's progress is that

A) they take less time and are less costly.
B) they increase the objectivity of ratings over those given by the people involved in treatment.
C) they ensure that there is no "regression to the mean" phenomenon.
D) they provide a valid prediction of how the client will behave in real life.