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: Radiology
Which of the following statements best describes 'Background Radiation'? 
Select and explain. 
A. Radiation in the background of nuclear reactors, B. Radiation in the background during radiological investigations, C. Radiation present constantly from natural sources, D. Radiation from nuclear fall out
C: Radiation present constantly from natural sources
Answer: C: Radiation present constantly from natural sources 
Explanation: Background radiation refers to those coming from the environment of natural radioactivity at the eahs surface and the those from direct cosmic radiation that arrives at the eahs surface. Ref: Radiation Biophysics By Edward L. Alpen, Page 432; The Biological Basis of Nursing : Cancer By William T. Blows, Page 54, 56