Remember, there are two theories of gravity: the graviton theory and the general theory of relativity. In the graviton theory, massive object exchange little particles called “gravitons.” This is just like electrically-charged objects exchanging photons (the electroweak force) and protons exchanging pions (the strong force). In this theory, then, the gravitational force is actually a FORCE. It is an interaction between two objects, and that interaction is governed by the exchange of gravitons.

The general theory of relativity says something quite different. It says that gravity is NOT A FORCE at all. There is no interaction between massive objects. Instead, what we experience as gravity is really just a consequence of how mass bends space. Thus, the moon and the earth do not really interact at all. However, since the earth’s mass warps space, the moon is forced to orbit the earth, because the moon “thinks” it is moving in a straight light, but that straight line is a circle because of the way the earth bends space.

Thus, if the general theory of relativity is true, gravity is not a force at all. The question says that it has been determined that there are only two forces: the electroweak force and the strong force. Since gravity is not on the light, gravity must not be a force, and that must mean general relativity is true.