You are answering a USMLE-style medical question. Question: During the normal catabolism of protein, urea and ammonia are produced as waste products. If these waste products are not cleared by the liver and kidneys, hyperammonemia can occur, leading to confusion and delirium. Fortunately, a healthy liver can clear these waste products via the urea cycle. Which of the following reactions is the rate limiting step in this cycle? Options: A) NH3 + HCO3- + 2 ATP --> carbamoyl phosphate + 2 ADP + Pi B) Carbamoyl phosphate + ornithine --> citrulline+ Pi C) Citrulline + aspartate + ATP --> arginosuccinate + AMP + PPi D) Arginine + H20 --> ornithine + urea Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.