Which Is a Disadvantage of Using an Active Comparator Arm

Which is a disadvantage of using an active comparator arm?

A) There are ethical concerns about denying treatment to some participants.
B) Active comparator arms are less useful than no intervention arms.
C) Use of comparators cannot establish how effective a treatment is over no treatment at all.
D) Active comparator arms are rare because they are not the best way to study a new treatment or therapy.