Answer:
D. changed this feeling into enthusiasm
Step-by-step explanation:
In the sentence, the word "apathy" refers to a lack of interest or emotion. The phrase "changed this feeling into enthusiasm" suggests that the narrator initially had a disinterested or indifferent attitude towards the theory and facts presented, but that this feeling was transformed into excitement or enthusiasm after reading and learning more.
Therefore, the phrase "changed this feeling into enthusiasm" helps to define the word "apathy" in the sentence as it contrasts the initial lack of interest with the later enthusiasm.