题目内容
据考古发现，我国陕西省半坡原始村落建在河谷阶地上，图甲为半坡原始村落复原模型，图乙为河谷阶地示意图。半坡原始村落选择建在河谷阶地上，考虑的主要因素有（） ①接近水源 ②安全防卫 ③对外联系便利 ④地势较高，免受洪涝之害
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在下图所示电路中，电源电压U=6V。若使电阻R上的电压U1=4V，则电阻R为（）。
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