What allows infant to develop the social smile

A. Stimulation from an Enriched environment allows infants to experience happiness

B. As an infant vision mature and as they continue to develop binocular vision, they are able to see other people faces, much more clearly, They then smile in response to the social stimuli.

C. The brain system associated with positive emotions are finally activated.

D. Infants do not develop stranger anxiety until later so they tend to smile at others before then