Macfarlane (1975) presented nursing newborns with two breast pads: one from their own mother and one from another mother. Two days after birth, newborns did not exhibit behavior demonstrating that they could tell the difference between the two pads

However, six days after birth, newborns turned toward their mother's pad more often than they turned to the other pad. In this example, learning occurred through the sensory system known as ___.
a. audition
b. vision
c. gustation
d. olfaction

D