Hildi wants to know if reduction of caffeine reduces anxiety in her client. For on month, she weekly measures her client's anxiety levels. She then asks her client to stop ALL caffeine consumption, and weekly measures her anxiety level (for one month). For the last month, her client return to consuming caffeine and again, anxiety is weekly monitored. The comparison between the average anxiety levels for all three months reveals the contribution of caffeine to the client's anxiety. What type of study is this? Question 4 options: case study longitudinal experiment ABAB study