You are answering a USMLE-style medical question. Question: A 62-year-old man with a history of chronic bronchitis comes to the physician because of a 1-month history of worsening shortness of breath and cough productive of thick sputum. He smoked one pack of cigarettes daily for 20 years but quit 5 years ago. Physical examination shows an increased anteroposterior chest diameter and coarse crackles in the lower lung fields bilaterally. Treatment with a drug that directly antagonizes the effects of vagal stimulation on the airways is begun. Which of the following drugs was most likely started? Options: A) Fluticasone B) Montelukast C) Tiotropium D) Cromolyn Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.