When two intermediate chemical equations are combined, the same substance that appears in the same phase can be canceled out, provided that

When adding chemical equations, a substance can be canceled if it appears on opposite sides of the combined equation: as a reactant in one step and a product in another. The same substance must be in the same phase so the two terms match exactly. Therefore, the correct choice is C, “it is a reactant in one intermediate reaction and a product in the other reaction.”

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