Extinction is a natural phenomenon. It is estimated that 99% of all species that ever lived are now extinct. Why then do we say that we are now experiencing an extinction (loss of biodiversity) crisis? A) Humans are ethically responsible for protecting endangered species. B) Scientists have finally identified most of the species on Earth and are thus able to quantify the number of species becoming extinct. C) The current rate of extinction is high and human activities threaten biodiversity at all levels. D) Humans have greater medical needs than at any other time in history, and many potential medicinal compounds are being lost as plant species become extinct. E) Most biodiversity hot spots have been destroyed by recent ecological disasters.