核心问题
The customs inspector was ____ for smugglers. A. on the guard B. on the watch C. on watch D. on alert(全国研考) 应选 B(on the watch) 还是应选 C(on watch)?
词典解答
on the watch (for) 的意思是“密切注视着”“警惕着”“对……留神”：
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Be on the watch; there are thieves about. 要警惕,周围有小偷。
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Our frontier guards are always on the watch for any enemy who might dare to intrude on our territory. 我边防战士时刻警惕着任何敢于入侵之敌。
on watch 的意思是“值班”：
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He was standing on watch. (Palmer) 他在站岗。
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Who's on watch now? (LDCE) 谁现在在值班?