I believe B is the correct answer. The sentence can be divided into one sentence that can stand alone: "She tripped over a backpack that had been carelessly left in the doorway." (independent clause). And a section that cannot stand as it's own sentence: "As she entered the auditorium" (dependent clause).
I want to say Suspicion because with the limited amount of test subjects, She may doubt her conclusions as there were not enough data or subjects to give an accurate result.
