Central pacific railroad built the transcontinental railroad eastward from Sacramento California by hiring 10,000 chinese laborers to work on the tracks
It was trench warfare that was used to a great extent in World War I. This led to a significant elongation of the conflict, since targeting soldiers was much harder.
The correct answer is actually D - Spying.
The reason why D is the correct answer is because during those times, musicians could be invited to parties, balls, and festive occasions that were hosted by one family or another family or nobility or any other important group of people who could pay them a hefty sum to tell them what another family was doing and vice versa. That is why their job offered them the opportunity to work as part-time spies.
