Where in an equation for photosynthesis does oxygen belong? O on the left side, because it is a product Oon the right side, because it is a product on the right side, because it is a reactant Oon the left side, because it is a reactant

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.