12．S45C中碳鋼，碳含量為 (A)0.045 wt% (B)0.45 wt% (C)4.5 wt% (D)45 wt%

2. A sphere with radius a contains a uniform volume charge density ofρvin the region: 0 ＜ r ＜ a. If the sphere is isolated by a conducting spherical shell with inner radius b and outer radius c placed concentrically, as shown in figure. What is the electric field intensity in the region b ＜ r ＜ c?(A)(B)(C)(D)(E)None of the above