The battle of Stalingrad was the turning point of Europe in WW2
It was considered a turning point, because at the time the U.S. was in the American-Spanish war. It took place from April to August in 1898. the war ended in victory for the United States and the end of the Spanish Empire in the Pacific and <span>Caribbean</span>. Only ~113 days after the outbreak of war, the Treaty of Paris, which ended the conflict, gave the United States control over the former Spanish colonies of Puerto Rico, the Philippines and Guam, and control over the process of independence of Cuba, which was completed in 1902.
Sources are used in a paper to show what ideas inspired you and whose theories you would be using and you cite the sources in order to avoid plagiarism. If you don't have sources, your paper will not be valid and you could get accused of plagiarism. The more sources you use the better but you should keep in mind that they need to be important for the paper and used with a purpose and not just to increase the number of sources.
The answer to this question is A
