A 62-year-old man is evaluated in the ER for sudden onset of swelling of the right calf with pain for past 2 days. The patient noticed the swelling after he woke up from sleep and had pain while walking. He cannot recollect any history of trauma, denies any recent travel, and has never had a blood clot in the past His medical history is remarkable for hypertension, adequately controlled with lisinopril, and stage-III squamous cell carcinoma of the lung for which he recently completed six cycles of chemotherapy. On physical examination, there is tender swelling of the right calf without erythema. His D-dimer is 640 ng/mL. Venous Doppler of the right leg reveals occlusive thrombi in the deep veins. A plan is made to initiate anticoagulation and to extend the treatment for 6 months.What will be the drug of choice for anticoagulation for this patient?