An index fossil, such as the Irish Elk, is :____________

a. the original set of fossil remains used to describe a species when it is first found.
b. a fossil with a strong stable isotope signature that can be used to calibrate the local vegetation cover.
c. the fossil remains of a species that existed during a well-established time period, to use in biostratigraphic dating.
d. the fossil of a species amenable to radiometric, and thus absolute, dating.