Which of the following compounds is used for water softening?
Ca3 (PO4PO_4PO4)2
Na3PO4Na_3PO_4Na3PO4
Na6P6O18Na_6P_6O_{18}Na6P6O18
Na2HPO4Na_2HPO_4Na2HPO4
(C) Na6P6O18Na_6P_6O_{18}Na6P6O18
Explanation: