question: As shown in the figure, the right triangle ABC and the equilateral triangle ABD are respectively drawn with the line segment AB as the edge, when the length of CD is the largest, the size of angle CAB is ()
  Choices:
  A:75°
  B:45°
  C:30°
  D:15°
query_wo: Please directly answer the question and provide the correct option letter, e.g., A, B, C, D.
  Question: As shown in the figure, the right triangle ABC and the equilateral triangle ABD are respectively drawn with the line segment AB as the edge, when the length of CD is the largest, the size of angle CAB is ()
  Choices:
  A:75°
  B:45°
  C:30°
  D:15°
query_cot: Please first conduct reasoning, and then answer the question and provide the correct option letter, e.g., A, B, C, D, at the end.
  Question: As shown in the figure, the right triangle ABC and the equilateral triangle ABD are respectively drawn with the line segment AB as the edge, when the length of CD is the largest, the size of angle CAB is ()
  Choices:
  A:75°
  B:45°
  C:30°
  D:15°
question_for_eval: As shown in the figure, the right triangle ABC and the equilateral triangle ABD are respectively drawn with the line segment AB as the edge, when the length of CD is the largest, the size of angle CAB is ()
  Choices:
  A:75°
  B:45°
  C:30°
  D:15°