Which of the following characteristics is essential to be considered an effective index

which of the following characteristics is essential to be considered an effective index fossil? selected answer will be automatically saved. for keyboard navigation, press up/down arrow keys to select an answer. a a. the organism lived in a very specific environment or two, such as tropical volcanic islands b b. the organism lived for a very limited period of geologic time c c. the fossils of the organism are exceptionally well preserved d d. the organism lived on the land, not in the ocean