Which of the following is (are) a difference(s) between a preferred provider organization (PPO) and a point-of-service (POS) managed care plan? (Check all that apply.) POS plans use "gatekeeper physicians" to control patients' access to care within the network while PPO plans do not. OPPO plans allow patients to receive care outside of the network of preferred providers while POS plans do not. PPO plans establish a network of preferred providers while POS plans do not. PPO plans require patients to select a primary care physician while POS plans do not