Lead surgeon Dr.Antonio Alcaraz instructs fellow surgeons and students watching the procedure through closed-circuit television.

VOA: standard.2010.05.30

So imagine the following example, an invader is thinking about invading a country, and there are two ways -- there are two passes if you like -- through which he can lead his army.

想象一下这样的例子,一个侵略者打算入侵一个国家,有两条路,想象成两个关口也行,侵略者必须通过一个才能进入

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