a. only happens above the species level
b. cannot happen to more than one species at a time
c. has happened to over 99% of species that ever lived on earth
d. happens when a species or population dies out
The correct answer should be C. has happened to over 99% of species that ever lived on earth. As a matter of fact, historically there have been over five billion species since they first started appearing millions of years ago, but 99% of them are now extinct, mostly due to things like the Great Extinction in the paleozoic era. Human caused extinction is nowhere near the numbers that the nature has reached.