alertness

例句：

1. 1. The driver was very alert when he saw the approaching truck.
   司机看到卡车时非常警觉。
2. 2. She is always alert for any danger that might be lurking around.
   她总是警觉地注意任何可能潜伏的危险。
3. 3. The police were alert to the possibility of a break-in.
   警方警觉到可能有入室盗窃的可能。