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, and point C is on ⊙O. If ∠A = 40.0, then the degree of ∠B is ()
  Choices:
  (A) 80°
  (B) 60°
  (C) 50°
  (D) 40°