Based on research with nonhuman primates, many researchers believed that there really was not much ...

Based on research with nonhuman primates, many researchers believed that there really was not much that is unique about humans' language skills. One challenge to this line of thinking is that ____.
 
  a. nonhuman primates were able to combine gestures into more and more complex and elaborate messages
  b. children use languages spontaneously and creatively, while nonhuman primates lack this tendency
  c. nonhuman primates never learned to combine words in systematic order
  d. nonhuman primates could generate complex messages once they had learned the basic symbols