Two concentric spherical shells have masses $M_1$ and $M_2$ and radii $r_1$ and $r_2$ $(r_1 < r_2)$. What is the gravitational intensity at a distance $r$ from the center where $r_1 < r < r_2$?

• A
  $I = \frac{G(M_1 + M_2)}{r^2}$ for $r < r_1$
• B
  $I = \frac{G(M_1 + M_2)}{r^2}$ for $r < r_2$
• C
  $I = \frac{GM_2}{r^2}$ for $r_1 < r < r_2$
• D
  $I = \frac{GM_1}{r^2}$ for $r_1 < r < r_2$