On that occasion they tested a theory that has been popular for several decades: that societies became recognisably “modern” in the mid-first millennium BC, during the so-called “Axial Age”, the period in which figures such as Plato, Buddha and Zoroaster appeared on the scene, promulgating moralising ideologies.
请问老师冒号后的that societies中的that不是引导主语从句吧,那这里为什么用that不用those呢?