Which of the following reaction is a redox reaction (a) P2O5 +2H2O gives H4P2O7 (b) 2AgNO3 + BaCL2 gives 2AgCL + Ba (NO3 )2 (c) BaCL2 +H2SO4 gives BaSO4 +2HCL (d) Cu +2AgNO3 gives 2Ag +Cu(NO3 )2