Milgram's obedience rate fell from 65% to about 21% when the experimenter gave instructions by telephone instead of standing in the room. Why is a change of that size, produced by so small a manipulation, the most important thing the study found?

B. Because it shows obedience is a property of the situation.

The 65% is quoted as a fact about human nature and the variations show it is a fact about one arrangement: telephone instructions drop it to 21%, a confederate refusing first collapses it to about 10%. A quantity that responsive is measuring the setup, and because it is situational, the variations double as a list of the levers.