When utilizing a transmission electron microscope, why is it necessary to stain the specimen with heavy metal salts?

In transmission electron microscope electrons are transmitted through an ultrathin section of a specimen to form a digital image.  It necessary to stain the specimen with heavy metal salts to create a contrast such that various cell sub structures are easily visible. For example, DNA is stained with heavy salts such that it is easily visible against the background.

radioactive decay occurs in unstable atomic nuclei – that is, ones that don't have enough binding energy to hold

the nucleus together due to an excess of either protons or neutrons.

i think so it’s will be b yes i believe its