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.
D. Contain the spread of Communism in every region of the world
The domino theory said that if one country falls to Communism, then the nations it associates with will turn to Communism as well. So, the solution for this was try to contain Communism so it wouldn't spread.
The closest one i know is A, hope this helped!
Like everywhere you go, though they may vary
