You are answering a USMLE-style medical question. Question: A researcher evaluates healthy breast tissue from 100 women, 50 women that were pregnant at the time of the study and 50 age-matched non-pregnant women. The breast tissue in pregnant women contained an increased number of acinar glands with epithelial proliferation compared to the non-pregnant women. Which process caused this change? Options: A) Hyperplasia B) Hypertrophy C) Atrophy D) Metaplasia Reply with ONLY the letter of the correct answer (A, B, C, or D). No explanation.