Question

alert
I
 /əˈlɚt/  adj   [more ~; most ~]
  : able to think clearly and to notice things
   An alert guard stopped the robbers.
   The nurse kept/stayed alert for any change [=watched for any change] in the patient's condition.
   He was tired and had trouble staying alert while he was driving.
   She wasn't mentally alert enough to answer the questions.
   an alert mind
   An alert watchdog guarded the door.
  — often + to
   The nurse should be alert to any change in the patient's condition.
   He is alert to [=aware of] his duties as a father.
  alert·ly  adv  
   A watchdog alertly guarded the door.
  alert·ness  noun   [  noncount  ]
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II
  noun  ,  pl   alerts
  1 [  count  ] : something (such as a message or loud sound) that tells people there is some danger or problem : an alarm or signal of danger
   They sounded an alert when enemy planes were approaching the city.
   Medical officials have put out an alert to hospitals to look out for the virus.
   The government has issued a terrorism/security alert.
  2 [  noncount  ] : the state of being ready for something you have been warned about (such as an attack)
  — used with on
   We need to be on alert for any sudden changes.
   The attack could come at any time, so the soldiers need to be on full alert.
   The recently flooded community is again on high alert as more rain is expected.
  see also red alert
  on the alert
   : looking for or expecting something (such as danger or an opportunity)
    The soldiers need to be on the alert at all times.
    — often + for
    When you're driving in winter you should always be on the alert for icy conditions.
    I'm always on the alert for a good bargain.
   
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III
  verb  , alerts, alert·ed, alert·ing [  + obj  ]
  1 : to give (someone) important information about a possible problem, danger, etc. : to warn (someone)
   Several neighbors alerted the authorities/police when they noticed strangers acting suspiciously.
   The governor alerted island residents that a hurricane was coming.
  2 : to make (someone) aware of something
   The teacher alerted the students that tests would be given the next day.
  — often + to
   A friend recently alerted me to the existence of a new museum in my city.
   We need to alert the public to the dangers of these chemicals. [=we need to tell the public about the dangers of these chemicals]