Two charged metallic spheres of same size repel each other by a force $F$. They are now touched with each other and are then separated to same initial distance. Now the force of repulsion is $F\text{'}$. Which of following are possible ?

In which of the following figures, electric field at point $O$ is non-zero?

Which of the four particles contribute to the electric field at point $P$ on the surface?

Which of the four particles contribute to the net electric flux through the closed surface?

A few electric field lines for a system of two charges $Q_1$ and $Q_2$ fixed at two different points on the $x$-axis are shown in the figure. These lines suggest that

Two nonconducting solid spheres of radii $R$ and $2R$ having uniform volume charge densities ${\rho }_1$ and ${\rho }_2$, respectively, touch each other. The net electric field at a distance $2R$ from the centre of the smaller sphere, along the line joining the centre of the spheres, is zero. The ratio $\frac{{\rho }_1}{{\rho }_2}$ can be: