question: Hint: Please answer the question and provide the correct option letter, e.g., A, B, C, D, at the end.
  Question: 如图，△ABC中，N是BC边上的中点，AM平分∠BAC，BM⊥AM于点M，若AB＝8，MN＝2．则AC的长为（）
  Choices:
  (A) 10
  (B) 11
  (C) 13
  (D) 12