An be inferred from the passage that: a. in an extinction event, there is a dramatic decrease in microbial life on earth. b. over 90% of easily observed, biologically complex species become extinct during a biotic crisis. c. the background extinction rate of animal and plant species is well below 50%. d. marine fossils are easier to find and thus more useful to those studying mass extinction events.

C. the background extinction rate of animal and plant species is well below 50%.

Explanation:The Reading Comprehension themed ”An extinction event (also known as a mass extinction or biotic crisis)” was the reference source for the answer.

Option C is correct, it requires that one joins two pieces of information. The start of the 3rd paragraph we see that “the five major mass extinctions have significantly exceeded the background extinction rate” and we later saw that ”the rate of those mass extinctions is over 50%.” This brings one to submit that the background rate of extinction is altogether, below 50%.

the answer is:

carbon dioxide

during the process of dna replication, brdu can replace thymidine and incorporate into the newly synthesized dna, cell proliferation thus can be reflected in the extent of brdu incorporation.