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In this task you will be given a text passage from a trivia quiz. You should choose the category this question belongs to from these categories: History, Science, Social Science, Fine Arts, Literature. You can only choose one category, so if in doubt, please choose the closest match.

They are 1,2-dipolar species of the type RmX+ - Y-Rn. If X is a saturated atom from the first row of the periodic table, then this type of compound is represented by a charge-separable form, but if X is an unsaturated atom doubly bonded to another first row element Z, the negative charge on Y may be stabilized by a pi conjugation. Defined as compounds in which an anionic site Y- is attached directly to a heteroatom X+ carrying a formal positive charge these are, FTP, what compounds whose phosphonium variety is also known as the Wittig reagent
Output:
Science