4. \( H _{2} O _{2} \) acts as reducting agent in
a) \( Na _{2} S+4 H _{2} O _{2} \rightarrow Na _{2} SO _{4}+4 H _{2} O \)
b) \( 2 Fe ^{3+}+2 H ^{+}+ H _{2} O \rightarrow 2 Fe ^{3+}+2 H _{2} O \)
c) \( Mn ^{2+}+2 H _{2} O _{2} \rightarrow MnO _{2}+2 H _{2} O \)
d) \( 2 NaOCl + H _{2} O _{2} \rightarrow 2 NaCl + H _{2} O + O _{2} \)
