Statement (A): Equal volumes of all gases at the standard temperature and pressure contain equal number of moles.

Statement (B): In a chemical reaction the substances that undergo a chemical change are called reactants. 

Statement (C): In chemical reaction the new substances formed during a reaction are called products

 Gay Lussac’s Law of combining volumes: when gases react, the volumes of the reactants and products are in the ratio of small whole numbers provided the volumes are measured at the same temperature and pressure.

$\mathrm{EX}: \underset{1 \mathrm{volume}}{{\mathrm{H}}_{2\left(\mathrm{g}\right)}}+\underset{1 \mathrm{volume}}{{\mathrm{Cl}}_{2\left(\mathrm{g}\right)}}\to \underset{2 \mathrm{volume}}{2{\mathrm{HCl}}_{\left(\mathrm{g}\right)}}$

Avogadro’s Law: Equal volumes of all gases at the same temperature and pressure contain equal number of molecules or moles.

$\mathrm{EX}: \underset{\underset{\mathrm{n} \mathrm{mol}}{1 \mathrm{volume}}}{{\mathrm{H}}_{2\left(\mathrm{g}\right)}}+\underset{\underset{\mathrm{n} \mathrm{mol}}{1 \mathrm{volume}}}{{\mathrm{Cl}}_{2\left(\mathrm{g}\right)}}\to \underset{\underset{2\mathrm{n} \mathrm{mol}}{2 \mathrm{volume}}}{2{\mathrm{HCl}}_{\left(\mathrm{g}\right)}}$

Reactant and Products: The substances that undergo chemical change in the reaction are called reactants. The new substances formed during the reaction are called products.

Ex: Magnesium + Oxygen → Magnesium oxide

$\mathrm{Mg}+{\mathrm{O}}_2\to \mathrm{MgO}$

Mg, ${\mathrm{O}}_2$ are the reactants and MgO is the product.