After sitting for hours on a long flight where movement is limited, people may experience some swelling in their legs and feet. Which of the following explains this phenomenon?

• A. Smooth muscle is not active, so lymph is not circulated effectively.
• B. Skeletal muscles in the legs and feet are inactive, reducing the rate of lymph flow.
• C. The respiratory “pump” from breathing is not as effective.
• D. The change in air pressure at high altitude significantly disrupts lymph flow, particularly in the lower body.