question: Hint: Please answer the question and provide the correct option letter, e.g., A, B, C, D, at the end.
  Question: 如图，AB是⊙O的直径，EF，EB是⊙O的弦，点E是FEB的中点，EF与AB交于点C，连接OF，若∠AOF＝40°，则∠F的度数是（）
  Choices:
  (A) 20°
  (B) 35°
  (C) 40°
  (D) 55°