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到了下午三点，他的注意力开始不集中了。

By 3 PM, his concentration began to ___flag.

搬着箱子爬了四层楼之后，他的体力开始不支了。

After hauling boxes up four flights of stairs, his strength began to ___flag.

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.