What is extinction and why does it occur?

Extinction is the dying out of an entire species. Extinctions have occurred regularly throughout Earth's history. At certain times, major climate changes, catastrophic events such as large meteorite impacts, or massive volcanic activity on earth have caused the Extinction of large numbers of species.

i think its white blood cells

i hope this

instead of copying and pasting things off of tests or work done on computer which im sure you did, try and read over the question carefully