Two small spheres of masses $M_1$ and $M_2$ are suspended by weightless insulating threads of lengths $L_1$ and $L_2$ The spheres carry charges $Q_1$ and $Q_2$ respectively. The spheres are suspended such that they are in level with one another and the threads are inclined to the vertical at angles of $\theta_ 1$ and $\theta_2$ as shown. Which one of the following conditions is essential, if $\theta_1 = \theta_2$?

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