The sentence describes the professor’s lecture, focusing on the content and how it affects students
The first blank relates to how the lecture was filled with historical referencesThe word "rich" (B) fits well because it means abundant or full of content, which suits a lecture packed with references
The second blank contrasts the first by describing the lecture’s difficulty levelSince the lecture is described as "challenging," the word "complex" (C) fits best because it suggests something intricate or difficult to understand
Let's quickly check the other
(A) sparse — means lacking content, which contradicts the idea of many references
(D) engaging — positive but doesn’t imply difficulty
(E) simple — contradicts "challenging"
(F) clear — also contradicts "challenging"
Therefore, the correct choices are (B) "rich" and (C) "complex"