Explanation

(c)
Sodium pyrophosphate is represented by Na_{4}P_{2}O_{7.}It is sodium salt of pyrophosphoric acid (H_{4}P_{2}O_{7} ). Which may be considered to be made up by two molecules of ortho phosphoric acid eliminating one molecule of H_{2}O.
2 H_{3}PO_{4}$\ \overset{- H_{2}o}{\rightarrow}\$ H_{4}P_{2}O_{7}
_{pyrophosphoric acid}
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