The option that presents the best meaning for "firsthand" in the context of the paragraph is "with their own eyes," as is further explained below. (option B)
"Firsthand" is an adjective used to refer to something we have experienced directly, ourselves, as opposed to something someone else told us about. In the paragraph, the word is used to talk about doctors encountering yellow fever cases.
Taking the context into consideration, the best option is letter B, "with their own eyes." No labs or blood samples are mentioned in the paragraph, and talking to other doctors would not be a "firsthand" experience, which eliminates the other options.
In conclusion, option B is the correct answer. "Firsthand", in this context, means "with their own eyes."
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