A solid uncharged conducting sphere has radius $3a$ contains a hollowed spherical region of radius $2a$. A point charge $+Q$ is placed at a position a distance a from the common center of the spheres. What is the magnitude of the electric field at the position $r = 4a$ from the center of the spheres as marked in the figure by $P?$ $\left( {k = \frac{1}{{4\pi { \in _0}}}} \right)$
  • A$0$
  • B$\frac{kQ}{16a^2}$
  • C$\frac{3kQ}{16a^2}$
  • D$\frac{kQ}{9a^2}$
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