used to indicate the spea-ker's firm belief that something is true even if evidence is not given or available
无疑地,确实地,我确信
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those who left were attracted, no doubt, by higher pay .毫无疑问,那些离去的人是受了高薪的吸引。
used to introduce a concession which is subsequently dismissed as unimportant or irrelevant
很可能,或许
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they no doubt did what they could to help her, but their best proved insufficient .他们或许尽力帮助了她,但还不够,他们没有全力以赴。