Marie is a high school teacher. She has to prepare a bar graph to show the salt constitution in seawater. Which salt should she label on the highest bar?

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.