According to research on infants' recognition of their mothers,

a. infants can recognize that mothers look different from strangers, beginning at less than 1 week of age.
b. because infants' vision is so primitive, they cannot visually recognize their mothers until they are about 6 weeks of age.
c. prior to birth, the walls of the uterus prevent the sound of the mother's voice from reaching the fetus.
d. infants do not seem to be able to distinguish their mother's voices from those of strangers until they are 4 weeks of age.

Ans: a