【評論主題】27. A: I saw Shannon having a drink with Tom at a pub lastnight.B: The girl you saw ______ Shannon.

【評論內容】表達對於過去的推測用語態助動詞的過去式，所以A跟B可以先刪掉，則選C是因為在否定的程度上could not have比must not have 更強烈。你見到的那位女孩所"幾乎可以確定是絕對不可能"是Shannon。 希望有幫到妳學姊:)