如图，将三角形ABC沿BC方向平移acm得到三角形DEF，若△ABC的周长为bcm，则四边形ABFD的周长为____ cm（用含a，b的式子表示）

“自然选择学说”可以解释生物进化的原因，它的提出者是（　　）

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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.

chūn      tiān      dào      le  ，  bǎi      huā      qí      fàng  。

____。