A father takes his baby out for a walk. The baby reaches over to touch a pink flower and is stung by a bumblebee sitting on the petals. The next day, the baby's mother brings home some pink flowers. The mother removes a flower from the arrangement and takes it over for her baby to smell. The baby cries loudly as soon as she sees it. According to the principles of classical conditioning, the conditioned stimulus in this scenario is the.

A. mother.
B. pink flower.
C. baby's crying.
D. bumblebee.