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    The skin is ____ essential part of your body and its ____ organ. You have three ____ of skin which act as a barrier ____ disease, poisons and the sun's harmful rays. The ____ of your skin are also very ____: it keeps you warm or cool; it prevents your body from ____ too much water; it is where you feel cold, heat or pain it gives you your sense of ____. So you can imagine, if your skin gets ____ it can be very serious. First aid is a very important first step in the ____ of burns.

阅读下面古诗词，完成问题。

海棠

（唐）郑谷

春风用意匀颜色，销得携觞与赋诗。秾丽最宜新著雨，娇娆全在欲开时。

莫愁①粉黛临窗懒，梁广丹青点笔迟。朝醉暮吟看不足，羡他蝴蝶宿深枝。

【注】愁：古代美女的通称。

菩萨蛮

（唐·五代）温庭筠

小山重叠金明灭，鬓云欲度香腮雪，懒起画蛾眉，弄妆梳洗迟。

照花前后镜，花面交相映。新帖绣罗襦，双双金鹧鸪。

如图，在Rt△ABC中，∠BAC=90°，∠ABC的平分线BD交AC于点D，DE是BC的垂直平分线，点E是垂足．已知DC=5，AD=2，则图中长为$\sqrt{21}$的线段有（　　）

第二次工业革命的核心内容是（    ）