Question

in the
experiment, neither the experimenter nor the
subjects knew which group was the control group.
options (as circles): double-blind, observed, conclusion

Explanation:

The question is about identifying the type of experiment where neither the experimenter nor the subjects know the control group. A double - blind experiment is defined as one in which both the researchers (experimenters) and the participants (subjects) are unaware of which group is the control group and which is the experimental group. "Observed" refers to noticing or recording data, and "conclusion" is the end - result or summary of an experiment, so they do not fit the context of the sentence.

Answer:

double - blind