Gibson and Walk Used Six-Month-Old or Older Babies in Their

Gibson and Walk used six-month-old or older babies in their research with the visual cliff, because

A) babies younger than six months old do not yet have depth perception.
B) the babies had to be able to crawl across the visual cliff.
C) younger infants are not yet attached to their mothers.
D) research ethics do not permit this kind of research with younger infants.
E) younger infants are unable to use kinetic cues.