黑客是网络系统常见的非法入侵者。

Hackers are common illegal intruders in network systems.

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只有非法入侵者从外向内闯入警戒区，才会触发报警。

Only the illegal invaders from the inside outward into the alert zone would trigger the alarm.

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就在这个时候，一对夫妇下楼来做晚饭，给两个非法入侵者提供了极好的下台阶的机会。

A couple walked downstairs to cook their supper at that moment; thus provided a marvellous chance for the intruders to step down.

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