Studies Using Sign Language or Lexigram Boards to Teach Chimpanzees

Studies using sign language or lexigram boards to teach chimpanzees language have found that

A) chimpanzees are able to associate symbols to meaning very quickly.
B) chimpanzees can combine words into complex forms but never master syntactic rules.
C) chimpanzees often focus their communicative efforts on abstract concepts.
D) chimpanzees communicate better using vocal sounds than symbols and pictures.