The United states fired a atomic bomb that destroyed some there thing and kill some of there people that is why.
I'm thinking d, since the model itself has factories/industries located on the ring just outside of the central business district, which is usually located in a city's center. Don't quote me though, it's been years since I've taken human geography.
Answer:
It is out of respect that they guard over the unknown soldiers and to make sure they will forever rest in the dignity and honor they deserve.
Explanation: