One- and Two-Day-Old Infants Received a Sucrose Solution

One- and two-day-old infants received a sucrose solution. The sucrose served as an unconditioned stimulus that elicited the unconditioned response of sucking. Milliseconds before the delivery of sucrose, the experimenter stroked the baby's forehead. Eventually, to the stroke of the forehead, the baby would suck. The conditioned stimulus is

A) the stroke of the forehead.
B) the sucrose alone.
C) the experimenter.
D) dextrose.