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Given a scientific passage and an answer, generate a question for the given answer.

Passage: Crossing charges. One method for obtaining subscripts in the empirical formula is by crossing charges. When crossing charges, you will sometimes find it necessary to reduce the subscripts to their simplest ratio to write the empirical formula. Consider, for example, the compound formed by Mg2+ and O2. Using the absolute values of the charges on the ions as subscripts gives the formula Mg2O2:. Answer: subscripts
Output:
Crossing charges is one method for obtaining what?