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, a teaching interest group wants to measure the height of a tree CD. They firstly measured the elevation angle of the tree top C at point A as 30.0, and then proceeded 10.0 along the direction of AD to point B, and the elevation angle of tree top C measured at B is 60.0  (the three points A, B, and D are on the same straight line), then the height of the tree CD is ()
  Choices:
  (A) 10m
  (B) 5m
  (C) 5√{3}m
  (D) 10√{3}m