It depends on which war you're referring to. I feel that the answer you're looking for is the radio, though.
The Pax Romana was a period of Roman peace, so through this they spread culture and ideas. Hope this helps! Happy Studying!
Source: I'm in AP World History :)
C. that's when the star spangled banner was made one of America's most prideful song today :)
It is D! Hope this helps you!