This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Epidemiological Study, 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. Odd's ratio is? 
Select and explain. 
A. 3, B. 6, C. 5, D. 10
B: 6
Answer: B: 6 
Explanation: ANSWER: (B) 6REF: Park 20th edition page 69 SmokerNon smoker Disease301040Non disease204060 5050 Odds ratio = (30 x 40)/ (20 x 10)= 6