Answer:
C
Explanation:
For the most part of the war, armies preserved old military tactics (fieldwork) of espionage rather than fully adapting to new techniques.
Here's why the other answers are incorrect:
A) "Photography and the telegraph brought some technological advances" in comparison to the previous wars, so A is incorrect. Intelligence was, for the most part, gathered in the same ways it had been previously, but these new technologies were still utilized.
B) Photograph and the telegraph are mentioned, but they are not the main point of the paragraph. Basically, "although the use of photography and the telegraph brought some technological advances," this fact is only being mentioned in the paragraph to emphasize the minimal progress of "the means of obtaining military intelligence," as intelligence was often still "gathered in simple ways" only. Armies "still relied on fieldwork." So B is incorrect.
D) Option D only refers to one source of information in the paragraph, "runaway slaves;" it does not fully or efficiently summarize the paragraph.
