All available sizes | 13 Kveta Pacovska, illus. for Jak Se Hraje Na Dvere a Jine Muzikantske Pohadky by Ilja Hurnik (Czech, 1973) | Flickr - Photo Sharing!
All sizes | 13 Tom Lewis, Developmental Psychology Today, 1971 (Diagrammatic outline of positive reinforcement...) | Flickr - Photo Sharing!
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