In Some Early Studies,researchers Attempted to Teach Chimpanzees to Speak

In some early studies,researchers attempted to teach chimpanzees to speak.What conclusions led to these studies being discontinued

A)  The reinforcement contingencies used in these studies were highly stressful for chimpanzees. 
B)  Chimpanzees lack the necessary vocal equipment to produce human speech sounds. 
C)  Chimpanzees lack the cognitive capacity to understand and produce language. 
D)  Humans are the only species capable of conscious communication.