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.
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解释
flag在此表示体力衰退、力气逐渐不足。
After hauling boxes up four flights of stairs, his strength began to flag.
flag在此表示体力衰退、力气逐渐不足。