Hey There!!
I think the answer to this is: The 3 one is. A Relative. The 4 one is. A noun. And, The last one is. adverbial.
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In the section "Pathology Indicates Manner of Communication," Snow explains the nature of cholera and argues that the study of the disease will help scientists understand how it spreads and how it can be averted. He combines his knowledge of the characteristics of the disease with his clinical experiences to build his argument.
He continues to build his case through other sections of the essay. For example, in "Cases Proving Person to Person Transmission," he provides evidence from other cases to support his theory that cholera is spread between people. Then, in another section, he maps an outbreak that occurred on Broad Street, connecting the affected individuals to a single water pump. After making these supporting arguments, he arrives at his main argument that cholera spreads through contaminated water.
In sum, Snow discusses a different concept in each section; these sections complement one another to build a complete, effective argument. The section labels help the reader follow Snow's line of thinking by mapping the main points.
