Which two structures perform similar functions in cnidarians and sponges?

spicules in cnidarians and pores in sponges
tentacles in cnidarians and nerve cells in sponges
stinging cells in cnidarians and cilia in sponges
gut in cnidarians and collar cells in sponges

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.