You tell me, man. One second he was fine, then out came the knife. I better get the guards.
我怎么知道？伙计！刚才还好好的呢，一会就拿上刀子了，我还是去找守卫吧。
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For example, security guards should patrol on campus around teaching buildings and student dorms, and enforce strict entrance checks, barring entry to anyone carrying a gun or a knife.