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.

there are treatment options for depression (mdd) that may work for you. one way of treating depression may be to affect the levels of certain chemicals that brain cells use to communicate, called neurotransmitters. scientists believe that certain neurotransmitters may be out of balance in depression.

although the exact way depression medications shown to ease overall depression symptoms in some people work is unknown, scientists think that some medications work by affecting the levels of a neurotransmitter called serotonin in the brain. other treatments work by affecting the levels of other neurotransmitters like norepinephrine or dopamine.