A researcher finds that when he smiles and makes eye contact with a participant, she becomes more responsive to his questions. He tries the same behavior on several others and finds that it also influences their responses to his questions. What type of single-case design did the researcher employ?

Across subjects multiple-baseline design

Explanation: The single case design involves experiment whereby only one group is used as to make inference in the study, the control group and treatment group aren't distinct. When behavior is measured or observed in two or more participants belonging to a single group by delivering a the same intervention or treatment at different poinf in time. In the scenario above, the smile and eye contact is the treatment being delivered or used on the participants to measure the the level of responsiveness of the participants. After trying the treatment on the first subject, the treatment was so test on the other participants, hence

answer; /// i do believe the correct answer is ( a