Slavery tolerated because of "necessity" means that you do not really want to do it, but you think you must do it. An example is when you have a farm and you need people to work on it. The trouble behind this scenario is that your farm isn’t making money to pay your farm helpers their proper wages, so you resort to slavery. In this case, you do not need money for salaries because your farm is doing well to keep your slaves clothed and fed.
Slavery affirmed as a "moral right" is when you think it is your right to be the master and the slaves deserves to be slaves.
By treaty with France after the victory in war
The correct answer to your question would be D.
The Rape of Nanjing was significant because of the sheer volume of people who were massacred, because it was actually a direct act of manipulation and terrorism, these two things made also the United States more attentive towards the happening in the Pacific as it saw that the Japanese had an increased growth in influence over this region.
Iron Joseph Paxton design the crystal palace
