195 countries reached an agreement(协议) on Dec. 12, 2015 after two weeks' talks in Paris. They talked about climate(气候) change. It is the first time in history that most of the world's countries, rich or poor, promised to take action to deal with climate problems.

    Below is a chart showing where the greenhouse gas(温室气体) comes from.

Where is greenhouse gas from?

    The Paris Agreement(巴黎协定) sets up a goal of cutting down the earth temperature to less than 2℃. If the warming gets above 2℃， there will be serious problems on the earth like natural disasters.

The agreement also asks countries to volunteer to do things. For developed countries, they will do even more. They have a plan to offer over 100 billion dollars to help poorer countries to develop clean energy like water and wind power.

    However, the Paris Agreement won't solve earth warming by itself. The future also depends on how well each country will carry out their promises.

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       眼前是几根埋在水里的枯木桩子，日久天长，也许人们忘记这是为什么埋的了。这里的水却是镜一样平，蓝天一般清，拉长的水草在水底轻轻地浮动。鬼子们追上来，看着就扒上了船。老头子又是一篙，小船旋风一样绕着鬼子们转，莲蓬的清香，在他们的鼻子尖上扫过。鬼子们像是玩着捉迷藏，乱转着身子，抓上抓下。
　　一个鬼子尖叫了一声，就蹲到水里去。他被什么东西狠狠咬了一口，是一只锋利的钩子穿透了他的大腿。别的鬼子吃惊地往四下里一散，每个人的腿肚子也就挂上了钩。他们挣扎着，想摆脱那毒蛇一样的钩子。那替女孩子报仇的钩子却全找到腿上来，有的两个，有的三个。鬼子们痛得鬼叫，可是再也不敢动弹了。
　　老头子把船一撑来到他们的身边，举起篙来砸着鬼子们的脑袋，像敲打顽固的老玉米一样。
　　他狠狠地敲打，向着苇塘望了一眼。在那里，鲜嫩的芦花，一片展开的紫色的丝绒，正在迎风飘撒。
       在那苇塘的边缘，芦花下面，有一个女孩子，她用密密的苇叶遮掩着身子，看着这场英雄的行为。

下图是我国西南地区某中等城市的功能区分布示意图。读图，回答下面小题。

生物体细胞的体积趋向于小的原因是（   ）