I hope they will finish the project in time, but they themselves are very that it will.A.

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A. certain
B. confident
(答案→)C. doubtful
D. suspicious
解析：doubtful  doubtful about/of/that...“对……疑惑的，不肯定的”；
be confident“有信心的”；be certain of/about/ that“对……确定的”; be suspicious of/that“对有怀疑，有疑心的”