This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Dental
The learning theory of digit sucking was proposed by 
Select and explain. 
A. Davidson, 1967, B. Sears and Wise, 1982, C. Johnson and Larson, 1993, D. Sigmond Freud, 1905
A: Davidson, 1967
Answer: A: Davidson, 1967 
Explanation: The learning theory: Davidson (1967)
This theory advocates that non-nutritive sucking stems from an adaptive response.
The infant associates sucking with such pleasurable feelings as hunger. These events are recalled by sucking the suitable objects available, mainly thumb or finger.