Why Would Researchers Design a Study in Which Neither the Participants

Why would researchers design a study in which neither the participants nor the investigators interacting with them know whether the participants have been assigned to a control group or to an experimental group?

A) In this design it is easier to analyze statistically the results of the study.
B) If the participants were given their choice of groups, the group memberships would likely be uneven.
C) The experimenters' expectancies might influence the participants' behavior.
D) Participation in the study would decrease if people had this knowledge beforehand.