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某品牌滚筒洗衣机,筒门处有开关S1,筒门关闭时S1闭合,筒门打开时S1断开.筒门开关S1和控制开关S2必须同时闭合时,洗衣机才能工作.下列符合要求的电路模型是( )
在农田生态系统中个,田鼠吃农作物,猫头鹰捕食田鼠,下列说法不正确的是( )