Answer :
Radioactivity kill cells, so let say you have your phone on but its in your pocket ok the radioactive from the phone is killing your cells.
It is essential to assume a constant rate of radioactive decay because it the rate of radioactive decay would have been changing continuously , then it would have been extremely difficult to find the age of a radioactive element or any component and structure that has radioactive element in it. It is fundamental assumption in radioactive dating
