Kisilevksy Et Al

Kisilevksy et al. (2003) showed that fetuses one to two weeks before birth recognized their mother's voice.This was accomplished by:

A) measuring differences in the infant's heart rate.
B) observing differences in motor behavior when the infant is in the presence of his or her mother or when the infant is in the presence of another woman.
C) training the infant in a conjugate reinforcement task.
D) retrospectively asking children about such experiences when they are much older.