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
Which of the following is a naturally occurring radioactive substance in the body in small quantities? 
Select and explain. 
A. Radium 226, B. Bismuth, C. Iodine 131, D. Potassium 40
D: Potassium 40
Answer: D: Potassium 40 
Explanation: Ans. is 'd' i.e., Potassium