On the basis of research with young children, a developmental psychologist hypothesized that the skills involved in copying curves must be developed before the skills involved in copying angles can be developed. Which one of the following, if true, supports the developmental psychologist's hypothesis? ['All of the children who can copy angles can also copy curves.', 'All of the children who can copy curves can also copy straight lines.', 'The ability to discriminate angles must be developed before angles can be copied.', 'Some of the children who cannot copy curves can copy angles.']