the right answer is "Two atoms of silver are needed to complete the reaction." because two atoms of silver are needed to complete the reaction. The silver atoms are used to produce silver sulfide. There must be two atoms of silver on each side of the arrow to balance the equation. Atoms are never lost or gained in a chemical reaction.
Although the complete chemical equation was not stated in the question, I can infer that Ag or silver is one of the reactants. This is because silver is not typically produced by chemical reactions, rather, it is sourced from mining silver ores. Also, the last option mentions the production of silver sulfide. This means that the chemical reaction referred to is: 2Ag + H₂S →Ag₂S + H₂. Thus, the 2 in 2Ag tells us that t<span>wo atoms of silver are lost in this reaction.</span>
