I can't figure this one out

In which research method does a researcher change the value of one variable to determine how the change affects another variable? a. experiment b. observational research c. survey d. case study

Flowchart with 6 boxes. box a sits at the very bottom of the left side of the flowchart, which flows into box b. box b flows into box c. box c flows into box d. on the bottom of the right side of the flowchart, box f flows into box e, and box e flows into box d. box d is the highest point in the flowchart. which of the following sections represent state-level courts? d, e, f b, e a, b, c a, f

As of 2013, the world’s total population was about