It would be necessary for a scientist to know both of these terms. Both controls and constants are variables that are used in experiments. During operation, a constant doesn't change. Examples of constants in experiments include when water freezes or boils, as well as the speed of light.
Control can vary but isn't kept constant to show the dependent and independent variables relationship. Being controlled allows the variables to be recorded and monitored to find their influence. Temperature, pressure, and time are examples of controls. It is common for an experiment to have more than one control.
When it comes to determining whether the speed of light is constant or not, there are a few things to be taken into account. Any change can make the speed of light faster or slower, depending on the variables that are present. If the molecular bonds are changed, the pace could change itself.
Every book will tell you that the speed of light is constant, however, there are new theories that are coming to light which are challenging this statement. Nothing stays constant forever in the universe. There are new stars, new moons, and other phenomena discovered each year. While planets undergo change, it would stand to reason that the universe itself goes through changes as well.
The study of astronomy and cosmology are assisting in determining whether the speed of light truly does remain constant, or whether there are variables that can either make it a bit faster or slower, depending on the environment.