Answer the following question given this paragraph: 

Even though large life forms have been very successful on Earth, most of the life forms on Earth today are still prokaryotes—small, relatively simple single-celled organisms. Fossils indicate that many organisms that lived long ago are extinct. Extinction of species is common; in fact, it is estimated that 99% of the species that have ever lived on Earth no longer exist.


Q: An estimated 99 percent of the species that have ever lived on earth no longer exist, showing that what event is common?

 Choices:

- accumulation

- over fishing

- pollution

- extinction

A:
extinction
