Why are isotopes that have relatively short half lives the only ones used in medical diagnostic tests

 Hence, it is less preferrable to choose an isotope to enter your body, which will emit radioactivity for a long time, so we tend to choose isotopes, the radioactivity of which ceases quickly, so that the least possible amount of damage is caused to the cells

Butterfly wingspan is 7.7

The answer to this is the 3rd choice!