Complete the missing components/variables given as x and y in the following reactions
(a) `Pb(NO_(3))_(2) (aq) + 2KI(aq) to PbI_(2)(x) + 2KNO_(3)(y)`
(b) `Cu(s) + 2AgNO_(3)(aq) to Cu(NO_(3))_(2)(aq) + x(s)`
(c) `Zn(s) + H_(2)SO_(4)(aq) to ZnSO_(4)(x) + H_(2)(y)`
(d) `CaCO_(3)(s) overset(x)to CaO(s) + CO_(2)(g)`