What would be the most likely threat to internal validity for children's depression scores on June 6, 1944, also known as D-Day?

Question

Quizplus · Accepted Answer

The most likely threat to internal validity for children's depression scores on June 6, 1944, would be the history effect. This is because D-Day was a significant historical event that, in addition to any other factors, could have affected the children's depression scores. For example, if a child had a family member or loved one who was directly impacted by the event, then this could have led to a temporary increase in their depression scores. This threat to internal validity is not applicable to the other options listed.

What Would Be the Most Likely Threat to Internal Validity