Attribution Theory: External vs Internal Attribution

Attribution theory explores how individuals interpret events and how this relates to their thinking and behavior. In particular, it addresses the ways people attribute causes to different actions and outcomes. Consider the following scenario:

Sarah is a student who just received a poor grade on her psychology paper. When discussing it with her friends, she comments, 'This happened because my professor is really tough and grades unfairly.'

Based on attribution theory, which type of attribution is Sarah making in her statement?