This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Respiratory system, Subject name: Medicine
[{'content': 'True statement about pulmonary embolism \nSelect and explain. \nA. Prompt heparin therapy will dissolve thrombi, B. Known risk factors include gastric cancer, AIDS, C. Most patients manifest dyspnea, pain, hemoptysis, D. Free floating iliofemoral thrombi are an indication for a caval filter', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Unfractionated Heparin UFH anticoagulates by binding to and accelerating the activity of antithrombin, thus preventing additional thrombus formation. UFH is dosed to achieve a target activated paial thromboplastin time (aPTT) of 60-80 s. The two principal indications for inseion of an IVC filter are (1) active bleeding that precludes anticoagulation and (2) recurrent venous thrombosis despite intensive anticoagulation. Ref Davidson edition23rd pg 620', 'role': 'assistant'}]