we must take two aspects into consideration,both of __ assoc

we must take two aspects into consideration,both of them associated with the safety of the students.= we must take two aspects into consideration,both of which are associated with the safety of the students.
空格里填them,因为both of them associated with the safety of the students是【独立主格结构】,即：代词+过去分词短语.

为您解答如下：
因为逗号之后不是句子（没有谓语动词），而是一个独立主格结构，所以填them，不能用which。