The two-layered sac surrounding the heart is the pericardium.
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Decomposer hope this helps
Answer:
Explanation:
Student A because it requires a hypothesis
Answer:
D) found in a limited time span
Explanation:
Index fossils are the remains of living organisms for which we have already established relative age. These are fossils that only existed during a limited period of geologic time and are easily identified, found in large volumes, and found in many different locations around Earth. Due to these factors, index fossils may be used as a guide for comparison in determining the relative age of an unknown object.
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