Why Are Organic Reefs Useful Paleoecological Indicators

Why are organic reefs useful paleoecological indicators?

A) Organisms only form limestone reefs in cold, tropic waters.
B) Organisms only form limestone reefs where there is a great abundance of mineral matter in the water.
C) Organisms only form limestone reefs in warm, clear, shallow water.
D) Organisms in the modern world only form limestone reefs between 30 degrees north and south of the equator.
E) Modern organisms only form limestone reefs in warm, clear, shallow water, between 30 degrees north and south of the equator.