You are answering a USMLE-style medical question. Question: The ribosome is an essential organelle that plays an important role in the synthesis of new proteins. The location of the ribosome within the cell can vary and provides a clue to the function of the particular cell. What ribosome location is essential for protein-secreting cells? Options: A) Rough endoplasmic reticulum B) Smooth endoplasmic reticulum C) Cytosol D) Nucleus Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.