on the alert
 保持警惕,處于戒備,提防著


 A·lert, n. Mil. An alarm from a real or threatened attack; a sudden attack; also, a bugle sound to give warning. “We have had an alert.”
 On the alert, on the lookout or watch against attack or danger; ready to act.

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 on the alert
      adj : watchful and prepared for action; "bird watchers on the
            alert for a rare species"; "cops were on the job and
            caught them red-handed" [syn: on the alert(p), {on
            the job(p)]