Which of the following is an example of a conceptual model explaining the function of the lungs in the human body to upper elementary students?

a) A detailed diagram illustrating the structure of the lungs and how oxygen is exchanged for carbon dioxide.
b) A physical model of the respiratory system that students can manipulate to observe airflow.
c) A written explanation of the process of breathing and the role of the lungs in supplying oxygen to the body.
d) A metaphorical comparison of the lungs to a sponge, soaking up oxygen and releasing carbon dioxide.