One Hundred Psychiatric Patients Were Randomly Assigned to One of Two

One hundred psychiatric patients were randomly assigned to one of two groups. Group A received a new drug in pill form. Group B was given identical-looking placebo pills. A panel of psychiatrists, who did not know which pill each participant received, evaluated all participants for level of agitation. In this study, experimenter bias was reduced by:

A) having experienced psychiatrists evaluate agitation.
B) having researchers who don't know who got which pill.
C) adding another placebo condition.
D) adding a therapy group.