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Test analysis: a according to the law of conservation of mass, the chemical formula of x can be determined as Ag 2 S. error. B. When the poison is detected by silver needle, the oxygen molecules in the air get electrons, and the valence changes from zero before the reaction to H 2 O-2 after the reaction. The valence is reduced. Mistake. C. In the reaction, the valence of Ag changes from zero valence of elemental Ag to+1 valence in Ag 2 S after the reaction, and loses electrons to become a reducing agent; The valence of H|S in H 2 S has not changed, so it is neither an oxidant nor a reducing agent. Mistake. D in the reaction equation, 4mol of 2 S is generated and 4mol of electrons are transferred. Therefore, 1mo 1Ag 2 S is generated, and 2mo 1 electron is transferred by the reaction. Correct.