Select the correct answer from the drop-down menu. SI units are the modern form of the: A. geological system of measurement B. metric system of measurement C. US Customary System of measurement D. Imperial units of measurement.

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.