Many cell phone companies use a two-leg billing system that bills each leg of the call separately.

许多移动电话运营公司都使用两档计费系统，分别对呼叫的每一档进行单独计费。

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Its model involves a two-mirrored system that directs sunlight down an optical rod onto a small (1 square centimeter) photovoltaic cell.

它的模型有一个双面镜像系统。该系统直接把阳光投射进一块小型(1平方厘米大小)光伏电池的光学感应棒上。

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The experimental evidence suggests that a particularly good way of activating the cell-killing arm of the immune system is to use two vaccines based on different antigens.

实验之中的种种迹象表明一个能够充分发挥免疫系统的抗病毒能力的方法，那就是同时使用两种能够辨认不同种类抗原的疫苗，一为主，一为辅，一种负责刺激免疫系统,使其做好攻击艾滋病病毒的准备;

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Two specific types of immune system cell that attack invaders - CD4 T-cells and natural killer cells - increased in number with age, with a higher rate of increase in women than in men.

BBC: Biological clue to why women live longer than men

Most cell phone handsets come with a game or two built into the system, but they're usually pretty low-tech or limited in scope, so serious gamers usually download their own.

FORBES: Dude, Ring Me Up A Game