Read the following passage and mark the letter A, B, C, or D to indicate the correct answer to each of the questions from 30 to 34  

     When drawing human figures, children often make the head too large for the rest of the body. A recent study offers some insights into this common disproportion in children's illustrations. As part of the study, researchers asked children between 4 and 7 years old to make several drawings of men. When they drew front views of male figures, the size of the heads was markedly enlarged. However, when the children drew rear views of men, the size of the heads was not so exaggerated. The researchers suggested that children drew bigger heads when they know they must leave room for facial details. Therefore, the odd head size in children's illustrations is a form of planning ahead and not an indication of a poor sense of scale.  

The main subject of the passage is ____________