You are answering a USMLE-style medical question. Question: Patients with the diagnosis of sickle cell anemia make a specific type of hemoglobin known as HgbS. This mutation results in the sickling of their red blood cells when exposed to inciting factors such as hypoxic conditions. Patients are often treated with hydroxyurea, which has which of the following direct effects on their hemoglobin physiology? Options: A) Decreases oxygen carrying capacity of hemoglobin B) Increases levels of fetal hemoglobin (HgbF) C) Decreases levels of HgbS D) Decreases levels of fetal hemoglobin (HgbF) Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.