What is three things mass extinctions have in common

One, a lot of mass extinctions are based on theories by scientist. Two, they all caused major changes in the Earth's systems (Ex. ecology, atmosphere, surface, and waters). Three, they all have theories on what caused it.

Also this is obvious but in all mass extinctions they wiped out a lot of lifeforms, 90% of anything that has EVER lived on the Earth is extinct. 

Hope this helps! :-)

Aand b should be the answe