30. According to the passage, why is Leonardo da Vinci’s research into the human anatomy so helpful to doctors? (A) The knowledge provided by his research helped doctors in Europe discover the principles of physics. (B) Medical schools in Europe did not provide doctors with any information about illnesses and injuries. (C) Before his research, doctors in Europe were not trained to draw the structure of the human body. (D) His research provided knowledge which allowed doctors to have a better understanding of the structure of the human body.