$A$ uniform vertical electric field $E$ is established in the space between two large parallel plates. $A$ small conducting sphere of mass $m$ is suspended in the field from a string of length $L$. If the sphere is given a $+q$ charge and the lower plate is charged positively,the period of oscillation of this pendulum is:

• A
  $2\pi \sqrt{\frac{L}{g}}$
• B
  $2\pi \sqrt{\frac{L}{g + (qE/m)}}$
• C
  $2\pi \sqrt{\frac{L}{g - (qE/m)}}$
• D
  $2\pi \sqrt{\frac{L}{[g^2 - (qE/m)^2]^{1/2}}}$