Which belongs in the area labeled X? are multicellular are heterotrophs reproduce using spores are decomposers

Ialready know the answers to these questions and they are on the attachment. but i just need to know why that answer is the correct one. plz asap! this is for a report that i need to email by 10!

Long-period comets come from the oort cloud particles in earth’s atmosphere material from the asteroid belt the kuiper belt

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.