The correct graph representing the variation of total energy$\left(E_{\mathrm{t}}\right)$ kinetic energy $\left(E_{\mathrm{k}}\right)$ and potential energy $\left(U\right)$ of a satellite with its distance from the centre of earth is 

The particles each of mass $M$, are located at the vertices of an equilateral triangle of side $a$. At what speed must they move if they all revolve under the influence of their gravitational force of attraction in a circular orbit circumscribing the triangle while still preserving the equilateral triangle?