Which of the following best describes the sources a historian uses to support an argument

Assuming that this is referring to the same list of options that was posted before with this question, the correct response would be "primary and secondary sources", since the historian uses secondary sources to give background information on the topic in question, and primary sources to make an original point that adds to the body of research. 

The league of nations was found in 1919 to keep peace after world war l.but it failed to prevent world war ll in 1939,so in 1945 the nations of the world set up a new organization,the united state (un).since then, the un has encouraged international cooperation and worked to resolve conflict