Answer:
Long term radioactive dating such as in dating rocks
Step-by-step explanation:
If we examine the graph carefully, we will notice that the number of atoms falls to zero after about 40000 years. This is quite a long time! Many rock formations were formed thousands of years ago and their ages can only be measured by an isotope that has an equally long half life.
Hence for an isotope with such a long half-life, we can effectively measure the age of a rock formation that contains the particular isotope in question. Rocks are composed of diverse chemical isotopes which are normally used to determine the age of the rock formation. Since rocks were formed thousands of years ago, these isotopes must be capable of having a half-life of thousands of years also in order to be useful in measuring the age of rocks.