Nearly all our food comes from the soil（土地）. Some of us eat meat, of course, but animals live on plants. If there were no plants, we would have no animals and no meat. So the soil is very necessary for life. The top of the ground is usually covered with grass or other plants. There may be dead leaves and dead plants on the grass. The waste matter（粪）from animals also falls on it.
   All soil needs food. If we do not give it any, the plants will be weak. Farmers found that animals' waste is the best food for the soil, but chemical fertilizers（化肥）are also very useful. The same crop（庄稼）should not be grown in the same place every year, it's better to have a different crop. A change of crop and the use of a good fertilizer will keep the land good.

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下列说法中表示天气的是（　　）

甲、乙两地相距40千米，在甲、乙两地之间沿直线架设两条输电线，已知输电线每千米的电阻为0.2欧．现输电线在某处发生了短路，为确定短路位置，检修员在甲地利用电压表．电流表和电源接成如图所示电路进行测量．当电压表的示数为3.0伏时，电流表的示数为0.5安，则短路位置离甲地的距离为（　　）