In Follow Up Studies,how Was Milgram Able to Manipulate the Level

In follow up studies,how was Milgram able to manipulate the level of obedience?

A) Obedience disappeared when participants were required to manually force the victim's hand onto a shock plate.
B) Obedience was higher when the experimenter informed participants that he took complete responsibility for any consequences to the learner.
C) Obedience was higher when the experimenter gave directions in person.
D) Obedience disappeared when the experiment was conducted by an ordinary person rather than the experimenter.