Which research method is most suitable for studying factors that influence behavioral change over time?

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

a. behavioral variables c. physiological variables b. treatment factors d. self-report variables

Suppose that five years from now, there is a blood test that assesses the likelihood of a person's committing suicide. It is likely that this test will measure the blood levels of ____

a. dexamethasone b. serotonin c. dopamine d. adrenalin