In a cube with side length a, identical point masses m are placed at the vertices a, b, and c. Another identical mass is placed at the center point d of the cube. What is the configuration of the masses in the cube?
1) All the masses are in a straight line
2) All the masses are in a plane
3) All the masses are at the corners of a regular tetrahedron
4) All the masses are at the corners of a regular octahedron
