What are the four threats to internal validity that are associated with participants?

What will be an ideal response?

(a) Selection–people in the groups to be compared differ from one another at the outset of the study.
(b) Maturation–people change over the course of a study either physically or psychologically
(c) Attrition (Mortality)–different types of people may leave a study before it is completed
(d) History–events not specifically associated with the treatment may affect the outcome for one group.