Plz answer

read the paragraphs from the section "billion-year-old bacteria."

antibiotics are prized for their ability to combat the microbes that make humans sick. however, most of them actually come from bacteria. for example, streptomycin, which has been used to treat tuberculosis and plague, is produced by the bacterium streptomyces griseus. this microbe was originally found in the dirt of a new jersey farm field.

bacteria have been fighting one another for billions of years — far, far longer than humans have been around. it's hardly surprising that they have developed all the best weapons. yet the vast majority of these microbes don't grow well under controlled laboratory conditions, making them difficult to study.

how is the central idea developed in these two paragraphs?

a
by providing background to support the conclusion that it is better to look for microbes in the environment
b
by summarizing the most common types of microbes that are making humans sick in many areas today
c
by illustrating the reasons why microbes should not be grown under laboratory conditions in any case
d
by elaborating on the prediction that new bacteria will continue to develop resistance to any new antibiotics