Answer:
B. a double-blind procedure.
Step-by-step explanation:
A double-blind procedure in research refers to a setting where neither the researchers nor the participants know who is getting which pill. This means that the researchers as well as the participants do not know that whether they are getting the actual treatment or whether they are in the control group. Hence, the given example is a double-blind procedure. However, in a number of scenarios this is not possible as the researchers have to collect data as per the difference between the two groups and have to analyse them separately.