Where does radioactivity come from?
a the decay of certain atomic nuclei
b the atom losing electrons
c radioactive material
d U-238

C. radioactive material

Explanation:

This exposure primarily comes from cosmic rays, radioactive material in the earth (such as uranium-238), ingestion of naturally occurring radionuclides in food (such as potassium- 40), and inhalation of radon gas. In the United States, the average background radiation dose is 300 mrem/yr.

the ozone layer acts as a filter for the shorter wavelength and highly hazardous ultraviolet radiation (uvr) from the sun, protecting life on earth from its potentially harmful effects. when the sky is clear, there is an inverse relationship between stratospheric ozone and solar uvr measured at the earth's surface.

a:

by the combination of an unlimited number of elements