What Is the Biggest Challenge Faced by Psychologists Who Study

What is the biggest challenge faced by psychologists who study infant perception?

A) Infants are already studied by pediatricians, so it isn't considered appropriate for psychologists to also study them.
B) Infant perception is a topic that is of little theoretical interest to psychologists.
C) Since infants can't speak, psychologists may not know whether infants can see or hear certain things.
D) Infants spend too much time eating and sleeping to be good research subjects.