Which of the following is NOT true about mass extinctions?

a. It happens when a species goes extinct.

b. It can be caused by gradual environmental change.

C. It can be seen in the fossil record.

d. It can be caused by a catastrophic change.

Prokaryotic and eukaryotic cells are different in many ways other than the presence or absence of a nucleus. if both archaebacteria and eubacteria are prokaryotic cells, then why are archaebacteria more closely related to eukaryotic cells than they are to eubacteria?