ExplanationThe blank is about the review committee’s opinion of the candidate’s thesis, and the trigger word but indicates that the committee holds the opposite view of the candidate’s view.
The candidate is confident that the thesis is valid, so the blank means not confident. Correct choices (C) and (F) mean doubtful, and produce equivalent, appropriate sentences. Choices (A) and (E) represent the candidate’s view, not the committee’s view. Choice (B) means soothed, not doubtful. Choice (D) means annoyed and goes beyond the context of the blank and is not supported.
Hence options C and F are correct!
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