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
Which of the following condition must be met for a valid student-t test between means of two groups: 
Select and explain. 
A. Number of observation must be same - no, B. Standard deviation must be approximately same - no, C. Mean should be approximately same - no, D. Sample must be small
D: Sample must be small
Answer: D: Sample must be small 
Explanation: None