You are answering a USMLE-style medical question. Question: A scientist observes a myocyte beating in cell culture. Which step is the most direct necessary component of relaxation for this cell? Options: A) Influx of sodium ions B) Influx of calcium ions from the sacroplasmic reticulum C) Influx of calcium ions from outside the myocyte D) Efflux of calcium ions Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.