Researchers at MIT are studying cataclysmic variables, which are binaries that orbit each other in under an hour. The rate of the decay of the radius of a binary system depends solely on the cataclysmic variables' respective masses, the radius of the system, the speed of light in a vacuum c, the universal gravitational constant G (read: big G), and a unitless constant. If the rate of decay of the binary system's radius is proportional to G (read: big G) cubed, what power of the speed of light in a vacuum is the rate of radial decay proportional to?