Which secondary source would the author use to acquire reliable data about da Vinci's employment with Sforza?

The author would use Encyclopedia Britannica as a secondary source to acquire reliable data about da Vinci's employment with Sforza.

Explanation: This is a secondary source because it is evaluating tons of data in order to draw a conclusion about the issue. The source is not performing new research or experimentation but is commenting on existing information out there. Since this issue is regarding a technology that is not yet fully developed and because the research is still new, it is more appropriately covered by a secondary source. As a quality secondary source, it is able to compile myriad factors and studies in order to draw an interpretation or make predictions that might otherwise be impossible to see using only an isolated experiment.