How many times in the history of earth has the number of species dropped drastically

Which mantle is under the crust the upper mantle or the lower mantle i think it's upper let me know if i'm right

One statements is an example of a scientific observation. another statement is an example of a scientific explanation. identify the correct statement for each category to illustrate how scientific explanations are inferred from scientific observations.

Which kingdom includes some organisms that have no nucleus and can live in an environment with an extremely high salt content