[单选] Phone signals are relayed between cell towers and cellphones via radio waves． This radio frequency—or RF—radiation is a type known as non-ionizing （非电离的）． Unlike X-rays， non-ionizing radiation does not deposit enough energy into cells to release electrons from atoms or molecules， producing ions． So it tends to be far less harmful than ionizing radiation， such as X-rays． But that does not mean radio waves might not cause harm．What is the paragraph mainly about？