Scientists Describe the Recent Decline in Global Biodiversity as Earth's

Scientists describe the recent decline in global biodiversity as Earth's sixth mass extinction.They are having difficulty determining exactly how rapidly species are disappearing, but most scientists fear they are underestimating the rate and number of species being lost.This underestimation would most likely occur because

A) an unknown number of species have not yet been discovered and described.
B) some species listed as extinct may not actually be extinct.
C) scientists are not looking for rare species that are likely to go extinct.
D) species losses due to economic uses such as overharvesting are not counted.
E) scientists have no measurements of background rates or past extinction rates.