What are two primary sources of natural background radiation?

Natural background radiation comes from two primary sources: cosmic rays and ground gamma radiation. Cosmic rays are high-energy particles that originate from outer space and constantly bombard the Earth, contributing to the natural radiation exposure we experience. These rays can penetrate the atmosphere and reach the surface, where they contribute to our overall radiation dose.

Ground gamma radiation, on the other hand, comes from radioactive materials present in the Earth's crust, such as uranium, thorium, and radium. These materials emit gamma rays as they decay, which can be detected and measured. Together, these two sources account for a significant portion of the radiation that humans are exposed to in their environment.

The other choices mention various forms of radiation, but they do not represent the natural, background sources like cosmic rays and ground gamma radiation do. Solar radiation, for example, primarily encompasses visible light and does not contribute significantly to ionizing radiation exposure. Millimeter and infrared waves do not fall into the category of ionizing radiation associated with natural background radiation. Similarly, X-rays and ultraviolet rays are forms of radiation but are not considered natural background radiation sources; they are often human-made or associated with specific processes.