One Reason That American Sign Language (ASL) Is Often Taught

One reason that American Sign Language (ASL) is often taught to primates in nonhuman language research is that

A) sign language is built upon gestures that the animals already understand
B) primates have trouble signing in languages besides ASL
C) the anatomy of the nonhuman primates' larynx makes it difficult for the animals to speak
D) attempts to train the animals to communicate using operant conditioning were originally unsuccessful