What variable has a direct effect or relation to another variable?

Which of the following can destroy the reliability and validity of the result of an experiment?

Happiness and other feelings are traits which can be considered as variables. Happiness and other feelings can be classified as _______.

A patient is being tested whether his body temperature will increase if a certain antihistamine is injected in his bloodstream. Which of these is the dependent variable?

This is a changing characteristic or attribute in an object or subject of the study.