Which of the Following Is an Example of Manipulation of the Independent

Which of the following is an example of manipulation of the independent variable?

A) in a study on the relationship between income and school performance,intentionally studying only schools with majority low-income students
B) in a study on the relationship between income and school performance,observing the natural variation in the school performance of children
C) in a study on the relationship between threats to masculinity and sexual harassment,intentionally varying whether male participants had their masculinity threatened
D) in a study on the relationship between threats to masculinity and sexual harassment,observing the natural variation in sexual harassment behavior