设ε和ε’分别为轴向受力杆的轴向线应变和横向线应变，μ为材料的泊松比，则下面结论正确的是（       ）。

单项选择题As shown in the figure, a spherical capacitor is formed of two concentric spherical conducting shells of radius and , containing two dielectrics characterized by and with the dielectric interface parallel to the spells. The radius of dielectric interface is . Find the capacitance between the spells. A、, and B、, and C、 D、

单项选择题A． B． C． D．