Answer:
Confounding variable
Explanation:
Confounding variable refers to the type of variable that can disrupt the validity of the experiment by giving a certain effect to the variable.
The experiment above was designed to measure mood elevation among the subjects, but the living situation among the subjects are really different among one another and cannot be controlled by the researchers.
The Allies included Britain, France, Russia, Italy and the United States. These countries fought against the Central Powers which included Germany, Austria-Hungary, Ottoman Empire and Bulgaria. Archduke Ferdinand, of Austria-Hungary, was assassinated by a Serb on June 28, 1914.