A plant makes its own food in its leaves . Water comes to the leaves through the roots . Air gets into the leaves through very small holes . The green coloring in the leaves uses the water and air to make the food for the plant . It also needs sunshine because a plant can make food only when the sun is shining.
   Animals and people could not live without green plants . They both eat plants . People and some animals also eat the meat of some animals and these animals eat plant

你知道郑和下西洋的最主要目的吗？（    ）

农村常常建横截面积为半圆形的全封闭塑料薄膜蔬菜大棚，如图所示，如图不考虑塑料薄膜接头重合及埋在土里的部分，那么搭建一个这样的蔬菜大棚需要用塑料薄膜的面积是（ ）

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阅读下列材料，回答问题。

材料一　八政：一曰食(农业生产)，二曰货(商品流通)，三曰祀，四曰司空，五曰司徒，六曰司寇，七曰宾，八曰师。

——《尚书·洪范》

材料二　以九职任万民：一曰三农，生九谷……五曰百工，饬化八材；六曰商贾，阜通货贿……

——《周礼·天官》

材料三　僇力本业，耕织致粟帛多者，复其身 (免徭役)；事末利及怠而贫者，举以为收孥。

——《史记·商君列传》

材料四　世儒不察，以工商为末，妄议抑之。夫工固圣王之所欲来，商又使其愿出于途者，盖皆本也。

——清·黄宗羲《明夷待访录》