Background radiation sources include radioactive materials in the Earth, outer space, and nuclear reactors, but not

• A. Primary X-ray exposure
• B. Cosmic radiation
• C. Terrestrial radiation
• D. Internal radiation

Answer: (A)

The key idea is distinguishing natural background radiation from artificial imaging exposure. Natural background radiation comes from three main natural sources: terrestrial radiation from radionuclides in the Earth’s crust, cosmic radiation from outer space, and internal radiation from radionuclides already inside our bodies. These are ambient, ongoing sources of radiation that people are exposed to without deliberately producing it.

Primary X-ray exposure, on the other hand, is produced by dental or medical imaging equipment for diagnostic purposes. It is an intentional, controlled, artificial exposure, not a natural background source. That’s why it’s the correct choice as not being part of background radiation.