Which Method of Estimating Extinction Rates Can Be Considered a Direct

Which method of estimating extinction rates can be considered a direct, rather than an indirect, method?

A) Comparing current extinction rates to a background rate determined from the fossil record
B) Using the species-area relationship to calculate likely species loss given habitat loss
C) Measuring actual number of species present in an area each decade for several centuries
D) Using changes over time in conservation status, e.g., from endangered to critically endangered