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
Cross-sectional studies are carried on 
Select and explain. 
A. Same individuals but of same age, B. Different individuals but of same age groups, C. Same individuals at different ages, D. Different individuals of different age groups
D: Different individuals of different age groups
Answer: D: Different individuals of different age groups 
Explanation: None