Consider the observation that many school shootings have been perpetrated by people with a history of playing violent video games. in this scenario, what could the third variable be?

The right option is "the shooters history of being bullied"

Explanation:

People with a history of being bullied turns out to be bullies or portray a high level of violent behaviors. There have been studies that shows a correlation between playing violent video games and violence. There have also been studies that suggest that people who have a history of being bullied will grow up being bullies and displaying several violent conducts.

in an effort to stem the tide of refugees attempting to leave east berlin, the communist government of east germany begins building the berlin wall to divide east and west berlin. construction of the wall caused a short-term crisis in u.s.-soviet bloc relations, and the wall itself came to symbolize the cold war.

answer; john stuart mill's principle of utility is your answer; /// good luck