Two identical tennis balls each having mass $m$ and charge $q$ are suspended from a fixed point by threads of length $l$. What is the equilibrium separation when each thread makes a small angle $\theta$ with the vertical?

• A
  $x=\left(\frac{q^{2} l}{2 \pi \varepsilon_{0} mg}\right)^{1 / 2}$
• B
  $x=\left(\frac{q^{2} l^{2}}{2 \pi \varepsilon_{0} m^{2} g^{2}}\right)^{1 / 3}$
• C
  $x=\left(\frac{q^{2} l}{2 \pi \varepsilon_{0} mg}\right)^{1 / 3}$
• D
  $x=\left(\frac{q^{2} l^{2}}{2 \pi \varepsilon_{0} m^{2} g}\right)^{1 / 3}$