Four identical particles each of mass m are placed at vertices of tetrahedron of side length a gravitational potential energy system of particles is __
a. -√2GM²/1 (2 - 1/√2)
b. - 2GM²/1 (2 + 1/√2)
c. -√2GM²/1 (√2 + 1/√2)
d. - 2GM²/1 (√2 - 1/√2)