Why Don't Scientists Know the Exact Number of Species on Earth

Multiple Choice

Why don't scientists know the exact number of species on Earth?

A) because about 10,000 new species are identified each year.
B) because species are becoming extinct at a rate that is too fast to count
C) because the system used to define what constitutes a species keeps changing
D) because historical records are inaccurate
E) Scientists do know. There are 1.8 million existing species on Earth