in 1924 America's National Research Council sent two enginee
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这里是be experimented upon sth(对.进行试验),upon=on(就).一般短语整体识记比较好.

Instead,the studies ended up giving their name to the “Hawthorne effect”,the extremely influential idea (that the very act of being experimented upon) changes subjects’ behaviour.

括号中的内容是修饰the extremely influential idea changes subjects’ behaviour.的主语部分的.