There may in fact be more than one. It is an argument. Because it is an argument, it is subject to evaluation and analysis. Note here that none of these questions ask if you like the argument or its conclusion. This part of the evaluation process asks you not for your opinion, but to evaluate the logic of the argument.
Holistic logic regards the piece as a whole.
This is a difficult question, and often requires outside information, such as information on how other historians were writing about the topic. Don't let the absence of that information keep you from using your historical imagination. When we learn more about the context in which those arguments were made, however, they start to make more sense. Things like political events and movements, an author's ideological bents or biases, or an author's relationship to existing political and cultural institutions often have an impact on the way history is written. On the other hand, the struggle to achieve complete objectivity also effects the ways people have written history.