Scientists Have Great Difficulty Determining How Rapidly Earth's Eukaryotic Species

Scientists have great difficulty determining how rapidly Earth's eukaryotic species are going extinct. This is primarily because

A) there are so many species that it is difficult to calculate gains and losses.
B) species are continually being discovered, and the total number on Earth is unknown.
C) the current trend of measuring microbial diversity complicates the calculations.
D) loss of habitat makes it more difficult to find and count remaining species.