According to the Big Bang theory, how many forces - and which ones - operated in the universe during the GUT era?
A. 2 forces: gravity and a single force that later became the strong, weak, and electromagnetic forces
B. 3 forces: gravity, the strong force, and the electroweak force
C. 2 forces: the strong force and the electroweak force
D. 1 force that represented the unification of all four forces that operate today