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Which element forms the basis of all organic organisms a carbon b oxygen c hydrogen d nitrogen

What is the difference between an igneous, sedimentary and metamorphic rock

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.