(单选题)设静电场中的电位<img src="data:image/png;base64,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" alt=""> 只依赖于柱坐标系中的坐标变量<img src="data:image/png;base64,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" alt=""> ，则无源区中的方程及其解的是（），式中A和B是待定常数