Read this excerpt from The Dark Game. Another thing that changed very little in the years between the wars was the means of obtaining military intelligence. Although the use of photography and the telegraph brought some technological advances, the craft of spying on the enemy's army still relied on fieldwork.

In the rest of the text, how does the author develop this central idea that spying during the Civil War still involved mostly fieldwork?

A by enhancing the reader’s understanding of which technological advances were used during fieldwork
B by changing the reader’s perceptions about how fieldwork could help the Union cause
C by including more detailed information about who did fieldwork for the Union and how it was done
D by adding complexity to the topic by raising the moral issues involved in fieldwork and other espionage