question: Hint: Please answer the question and provide the correct option letter, e.g., A, B, C, D, at the end.
  Question: As shown in the figure, AB is the diameter of ⊙O, point C is a point on ⊙O, ∠C = 20.0, then the degree of ∠BOC is ()
  Choices:
  (A) 20°
  (B) 30°
  (C) 40°
  (D) 60°