新東方新托福聽力特訓Emily Carr

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https://online1.tingclass.net/lesson/shi0529/0000/859/24.mp3

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What does the professor mean by saying this?
A He is sure that some slides are missing.
B He thinks it possible that some slides are missing.
C He is sure how slides can disappear from the library.
D He thinks that some slides are just misplaced in the library.
(B)