Check all that are the types of fossils we have examined
1. trace
2. crustaceans
3. index
4. body

I believe the answer to your question is 3. Index

Hope this helps ya girl! :)

answer; the correct answer is desert plants;

the sugar-phosphate backbone forms the structural framework of nucleic acids, including dna and rna. this backbone is composed of alternating sugar and phosphate groups, and defines directionality of the molecule.