I must (道歉) for not having been able to write to you sooner.apologize

发布时间：2020-08-17 13:32:58

I must     (道歉) for not having been able to write to you sooner.apologize 

网友回答

(答案→)apologize 
解析：用原形，因为是用作情态动词的后面。
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