By 1915, started to be used in the war; they were originally used for reconnaissance; then they attacked enemy

The answer is airplanes.  They were originally used for reconnaissance but when they outfitted with machine guns, they were given a more important role of attacking enemy fortifications.  Apart from machine guns, they also employed bombs that were dropped on enemy trenches.

the real answer is b

the first two years of american revolution was not very encouraging for the patriots since the british were far superior in sea power, arms and troops.