To accurately complete the explanation, the chosen words should indicate that a new substance was formed and that observable changes occurred that are typical indicators of a chemical reaction. Here is one way to complete the sentences:
The mixture produced a new substance, as shown by the properties of Material C. The change in color and the change in mass are both signs that a chemical reaction did occur.
Feel free to replace "properties" and "mass" with words that are more relevant to your specific situation if needed!