How Do Physicists Know That the Fundamental Forces Become Unified

How do physicists know that the fundamental forces become unified as the energy of particle interactions increases?

A) The theory that says that they do is a very elegant one, and it would be nice if it were right.
B) Experiments have actually been done with high-energy particle accelerators where all four forces have been unified.
C) Although it is not possible to see unification in the laboratory, the theory of unification agrees with many of the observed properties of the universe.
D) Physicists have actually seen the start of the unification in high-energy particle accelerators, where the electromagnetic and weak forces become unified.