What does the law of conservation of mass say?

The law of conservation of mass say Physical transformation or chemical reaction could not create or destroy mass because it is an isolated system.

Explanation:

The system’s mass doesn't change after some time and nor any amount can be included or evacuated. In this manner, the amount of mass is preserved with time. This idea of preservation of mass is utilized in fields like mechanics, science, and liquid elements.

Mass of product = Mass of reactants. This states the law of conservation of mass.

As per the law of conservation of mass, the mass should remain constant over time. Moreover, in a chemical reaction, the mass of the products should be equal to the mass of reactants.  

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it is b that is the answer.