Any type of a close and long-term biological interaction between two different biological oragnisms

Explain how paleontologists use trilobite fossils as index fossils for various geologic time periods.

What is the one part of the nucleotide  that differs among the other different nucle  tides?

What is a difference between systemic and pulmonary circulation? a. systemic circulation carries oxygenated blood to the lungs and pulmonary circulation carries deoxygenated blood to the body. b. systemic circulation carries deoxygenated blood to the lungs and pulmonary circulation carries oxygenated blood to the body. c. systemic circulation carries deoxygenated blood to the body and pulmonary circulation carries oxygenated blood to the lungs. d. systemic circulation carries oxygenated blood to the body and pulmonary circulation carries deoxygenated blood to the lungs.