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
According to Mahler's theory, the period of childhood is divided into: 
Select and explain. 
A. Normal autistic phase, B. Normal symbiotic phase, C. Separation industrialisation phase, D. All of the above
D: All of the above
Answer: D: All of the above 
Explanation: According to Mahler's theory, the period of childhood is divided into three stages.

Normal autistic phase (0 - 1 year)
Normal symbiotic phase (4 wks to 4 yrs)
Separation individualization phase (5 to 36 months)