In a carefully designed study, researchers where study participants

In a carefully designed study, researchers where study participants
were randomly assigned to groups found that the group exposed to the intervention and the group that received a placebo intervention had similar positive response when tested at the conclusion of the study.
What is the best explanation of this result.

Options:

1) The researchers were not very experienced and did not know how to conduct a study.
2) The control group might have found the attention they received by being in the study an intervention with a positive effect.
3) The groups must not have had equivalent scores on the pre-test?
4) either/both responses 2 and 3

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