What element is added to toothpaste and city water systems to tooth decay?

Why are people not able to scuba dive in the deep part of the ocean

Skills of homo sapiens were found an excavation. the skulls were preserved because the bodies were frozen. so, these fossils are (blank) fossils.the image shows the evolution of skulls beginning 2 to 3 million years ago. based on the image, modern human skulls(blank) ape skulls.

The identities of substances are the same before and after which type of change