I'm not 100% sure but it could be: Voluntary Exchange takes place between buyers and sellers:
(btw I copied and pasted from the question so I didn't take any content :D)
If Robert McNamara is one of the options, then he is the correct answer - he was important in the Vietnam War, but not during the WWII.
I believe it's A.) A means of communication. It depends on what you're using it for in a sentence. Hope this helps! :))
It marked the start of the American War of Independence
It was mainly developments in the "steel" industry that made important contributions to improvements in the transportation system in the United States during this time, since the major advancement in transportation was in the railroads, which were made from steel.
