In Which Variation of the Milgram Research Were Participants MOST

In which variation of the Milgram research were participants MOST likely to obey the experimenter's commands?

A) when the learner was in another room and the experimenter was in the room with the participant
B) when two other participants refused to comply with the commands
C) when the learner was in the same room as the participant
D) when the experimenter wasn't in the room but gave instructions by phone