Detailed Instructions: In this task, you are given a context and four options. Each option is a suggested ending for the context. You should read the context and pick the best ending for the context. Please answer with "A", "B", "C", and "D".
Q: [header] How to measure oxygen saturation using pulse oximeter [title] Understand the relationship between oxygen and the blood. [step] Oxygen is breathed into the lungs. The oxygen then passes into the blood where the majority of the oxygen attaches to hemoglobin. <sep> (A) [substeps] Oxygen is produced as the presence of oxygen in the body. As long as the body does not expend oxygen in the blood, of course, there is no greater amount of oxygen coming to the hemoglobin, and a large percentage of the oxygen is produced in hemoglobin. (B) Hemoglobin is a protein located inside our red blood cells that transports the oxygen through the bloodstream to the rest of our body and tissues. In this way, our body is given the oxygen and nutrients it needs to function. (C) This may seem like a very useful exercise if you find that oxygen condenses below or above the heart. The difference in oxygen concentration between oxygen and the hemoglobin is that oxygen is concentrated. (D) [substeps] One study showed that oxygen intake maximizes oxygen critical to the body's digestion, which aids in stabilizing and regulating cardiovascular function (as the body uses oxygen). [title] Observe the amount of oxygen in a given menstrual cycle.
A:
B