Internal costs are easy to see and explain. They are costs that a business bases its price on. They include costs like materials, energy, labour, plant, equipment and overheads. External costs are costs that are NOT included in what the business bases its price on.
As the earth has active tectonics we know that rocks are recycled and the oldest rocks may therefore just be the oldest rocks that have not been recycled yet. We know the true age of the earth by looking at radioactive isotopes of things like spacedust and meteors that formed at the same time as the earth as they have not been affected by the tectonic process of recycling here on Earth.
Answer:
hereditary variat on selection is made of those genetic variats that well be passed on most effectively to the following generation
