The answer is: The source is reliable and knowledgeable.
Remember that in an informative essay, the writer's opinion is not given. The writer objective is not to persuade the reader, the main purpose is to educate him/her on certain topic. As it is an academic paper from a high school or college level it requires a source that characterizes for it's reliability. Although this sources can be found in libraries or the internet, they are not exclusive of these type of media.
