What Is a Challenge to the Equivalency Design? Select All

What is a challenge to the equivalency design? Select all that apply.

A) The shortcoming of the equivalency design is that participants must be receiving an existing treatment that is believed to be or has been shown to be effective.
B) The goal of equivalency studies is to prove that differences between the treatments does not exist, and this is impossible.
C) Random error can hide a difference between the two treatments.
D) In equivalency studies, it is difficult to determine how much difference between the arms will be considered equal.