A researcher wants to determine if preschool attendance is associated with high school graduation for low-income students. She randomly assigns low-income children to two groups; one group will attend preschool program, the second group will not attend preschool. The researcher plans to follow the children in the study for 20 years and observe whether or not they graduate from high school. Which of the following is the explanatory variable in this study?

Whether or not a subject graduates high school
The income status of the children
Whether or not a subject attends preschool
The length of time it takes a subject to graduate high school