Which Is the BEST Reason for Using a Placebo as Part

Which is the BEST reason for using a placebo as part of the experimental design?

A) Because it makes it impossible for researchers to know who is receiving an intervention and who is not.
B) Because the only way to know if the treatment works is to compare it to a placebo.
C) Because it prevent subjects from knowing whether or not they are receiving the treatment.
D) Because it allows the incorporation a larger number of subjects in the experiment.