Answer
13. c. Both a and b
The law of conservation of mass states that in a chemical reaction mass is neither created nor destroyed. This implies the mass of the reactants is equal to the mass of the products.
14. Based on the class set of data provided in the table, it can be concluded that what happened to the mass of the system is: b. The mass decreased.
15. The best explanation for what was observed is: c. The mass went down because the bubbles contained a gas that left the system.