12. 產生一均勻磁場的磁向量位置為位置向量）可表示為？ (A)(B) (C) (D)(E)

12. A conducting spherical shell of inner radius a = 0.2 m and outer radius b = 0.25 m has a total charge of 5.0 μC on its surface. A point charge of -6.0 μC is placed at the center of the shell. Determine correct item for the electric field E in the regions: (A) a ＜ r , E = 1.0μC/(4πεor2) (B) a ＜ r ＜ b , E = 0 (C) r ＞ b , E = 6.0μC/(4πεor2) C/(4\pi\varepsilon_0 r^2) \)