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: Social & Preventive Medicine
In a study done to establish smoking as a risk factor for a disease, 30 out of 50 smokers developed the disease while 10 out of 50 non-smokers developed the disease. Odds ratio is? 
Select and explain. 
A. 6, B. 3, C. 5, D. 10
A: 6
Answer: A: 6 
Explanation: None