Read this paragraph 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. For the most part, intelligence was gathered in simple ways. Prisoners of war and deserters were interrogated. The Union army was always interested in hearing what runaway slaves had to report. And, of course, both armies relied on cavalry scouts, signal intercepts, visual observation, and captured correspondence.

What is the best summary of this paragraph?

During the Civil War, intelligence was gathered in the same ways it had been gathered in previous wars.
Photography and the telegraph were some of the technological advances that were used in Civil War espionage.
Most espionage during the Civil War was fieldwork that included observation, scouting, and interrogation of prisoners.
Enslaved persons that ran away, as well as prisoners from the Union Army, were good sources of information during the Civil War.