Imagine you're a doctor and a patient comes to you because of pain in their knee joint. You find that there is not enough synovial fluid present in their knee's synovial cavity, leading to friction when the knee moves. Which of the following structures in the knee joint is not functioning correctly?

• A. Bursae.
• B. Articular cartilage.
• C. Articular capsule.
• D. Synovial membrane.