You are answering a USMLE-style medical question. Question: A 72-year-old woman is admitted to the hospital for treatment of unstable angina. Cardiac catheterization shows occlusion that has caused a 50% reduction in the diameter of the left circumflex artery. Resistance to blood flow in this vessel has increased by what factor relative to a vessel with no occlusion? Options: A) 64 B) 16 C) 8 D) 4 Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.