Chimpanzees Are Intelligent Primates and Can Develop Some Rudimentary Language

Chimpanzees are intelligent primates and can develop some rudimentary language skills,albeit mostly evidenced through sign language and not speech.What is the primary barrier that prevents chimpanzees from learning to speak

A)  They do not have the appropriate vocal apparatus. 
B)  Their main form of communication is gestural, rather than vocal. 
C)  They cannot focus their attention on a complex cognitive task like word-learning. 
D)  They cannot grasp the symbolic aspects of human language.