alert [əˈlɜ:t]

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adj.机警的，活泼的，机灵的；vt.使...警觉

quick to see , understand and act in a particular situation

Drivers must be on the alert for traffic signals. 驾驶员必须密切注意交通信号。
The rabbit seems to be very alert all its life. 兔子似乎一生都小心翼翼，十分警觉。