In Milgram's Original Obedience Study, What Was the Voltage Level

In Milgram's original obedience study, what was the voltage level at which some participants first refused to continue shocking the "learner"?

A) at the 45-volt level, the level at which the sample shock was given to the "teacher"
B) at the 150-volt level, which was labeled moderate shock
C) at the 315-volt level, which was labeled extreme intensity shock
D) at the 420-volt level, which was labeled danger: severe shock