Answer: Hello your question lacks the required options but I will provide a more general answer
answer : Ineffectiveness/failure of the experimental Treatment given to the 50 subjects in the Treatment group
Step-by-step explanation:
The Placebo treatment is a treatment given for the purpose of psychological benefit. it does not have a therapeutic effect on the individual.
Given that there is very little difference between the success rate of the group whom actually receive the treatment ( for the purpose of therapeutic effect ) and the group that received placebo treatment we can conclude that the success of the Placebo group is due to the ineffectiveness of the Treatment given to the 50 subjects in the Treatment group