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
Small brown spots on teeth of a child, less than 4 years old are commonly due to: 
Select and explain. 
A. ECC, B. Deep caries, C. Staining, D. Fluorosis
A: ECC
Answer: A: ECC 
Explanation: None