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
Spot map was devised by ? 
Select and explain. 
A. Louis pasteur, B. Ronald Ross, C. David Moley, D. John snow
D: John snow
Answer: D: John snow 
Explanation: Ans. is 'd' i.e., John snow John Snow (1813-1858) found the link between cholera and contaminated drinking water in 1854, using spot map.