According to the law of conservation of mass, which statement about chemical reactions is always true?
A. the states of the substances in the reactants is the same as the states of the substances in the products
B. the number of atoms of each element in the reactants is the same as the number of atoms of each elements products
C. the number of substances in the reactants is the same as the number of molecules in the products
D. the number of molecules in the reactants is the same as the number of molecules in the products