How are scientists able to use fossils of extinct organisms as evidence?

How does the author's purpose in this passage differ from an author writing about her personal struggle living with asthma?

Moving down the ramp from 6 meters to

Which of the following must be true for a fossil organism to be useful as an index fossil? a. the fossil organism must be widespread geographically. b. the fossil organism must be abundant. c. the fossil organism must be limited to a short span of geologic time. d. all of the above.