Constants are factors that are not changed in the experiment. Since the experiment is testing different colors of light on plant growth, light color is being changed, and is therefore not a constant. Hope this helps! :)
CO2 or carbon dioxide is an example of a compound element. A compound element is formed into a new substance when two or more elements are chemically combined. One of the most common examples is CO2 or carbon dioxide. The elements combined are carbon and oxygen.
A proton has a positive (+1) charge, while a neutron is electrically neutral (carries no charge).