讲座进行到第二个小时时，观众的兴趣开始消退。
Audience interest began to flag during the second hour of the lecture.
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解释
flag在此指兴趣或热情的减退，听众不再积极投入。
Audience interest began to flag during the second hour of the lecture.
flag在此指兴趣或热情的减退，听众不再积极投入。