B. It reunited northern and southern China in the sixth century.
The Sui Dynasty was a relatively short one, it only lasted from 589 AD to 618 AD. Therefore, A. C. and D. are incorrect because the Dynasty was not in power at those times. So, B is correct because it did reunite northern and southern China with its creation in the 6th century.
It's asking why would the Nazi's build their camps away from places with lots of people
the answer should be along the lines of "so that they would be better hidden"
the answer is: Truth is only achievable through a process of inquiry and observation.
Agriculture was key to the development of modern "civilization" because by being able to grow food rather than getting it through trade or hunting, people were able to stay in one place instead of having to be where the food was. This also led to jobs because people became highly skilled in growing crops.