How Do Mass Extinctions Differ from Background Extinctions

How do mass extinctions differ from background extinctions?

A) Mass extinctions account for most species that have gone extinct in the history of life on Earth.
B) It is now recognized that mass extinctions but not background extinctions are generally caused by asteroid impacts.
C) During mass extinctions a large proportion of organisms that were alive at that time die out.
D) Background extinctions don't lead to great numbers of species lost.