Stanly Milgram of Yale University tested 40 subjects from al

Stanly Milgram of Yale University tested 40 subjects from all walks of life for their willingness to obey instruction given by "leader" in a situation in which the subjects might feel a personal distaste for the actions they were called upon to perform.
WHICH 后面的就好了 大意我都知道 特别是they were called upon to perform.UPON这个介词的用法说一下.