Why Is It So Difficult to Estimate How Many Species

Why is it so difficult to estimate how many species of insects there are on the Earth?

A) There is no central repository for information on new species on Earth.
B) New species continue to be discovered and other species go extinct.
C) Different methods of estimating numbers yield widely varied results.
D) All of these are valid reasons.
E) No estimate is necessary because a good approximation of the number of insect species is sufficient.