This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Environment and health, Subject name: Social & Preventive Medicine
Which of the following is a naturally occuring radioactive substance in the body in small quantities? 
Select and explain. 
A. Radium 226, B. Bismuth 60, C. Iodine 131, D. Potassium 40
D: Potassium 40
Answer: D: Potassium 40 
Explanation: Sources of radiation exposure are: Natural sources and Manmade sources. The natural sources include cosmic rays, environmental (terrestrial and atmospheric) radiation, internal radiation (Potassium - 40 & Carbon -14) Man-made Radiation: Medical and dental: X rays, radioisotopes Occupational exposure Nuclear Misc: TV, Radio-active dial watches, Luminous markers Park's Textbook of Preventive and Social Medicine, 25th Edition, Pg 803