You are answering a USMLE-style medical question. Question: A 35-year-old man comes to the emergency department with acute shortness of breath that developed after a 10-hour international flight. His pulse is 124/min and pulse oximetry on room air shows an oxygen saturation of 90%. He weighs 50-kg (110-lb). A diagnosis of pulmonary embolism is suspected and intravenous heparin is initiated. If the volume of distribution of heparin is equivalent to 60 mL/kg and the target peak plasma concentration is 0.5 units/mL, which of the following is the most appropriate loading dose for this patient? Options: A) 3,000 units B) 1,500 units C) 6,000 units D) Cannot be calculated, as bioavailability is not known Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.