Which of the following oxides can act both as an oxidising agent as well as a reducing agent?

When ammonia is passed over heated copper oxide,metallic copper is obtained. This reaction shows that ammonia is a.......

Consider the reactions:
$(a)$ $H_3PO_{2(aq)} + 4AgNO_{3(aq)} + 2H_2O_{(l)} \rightarrow H_3PO_{4(aq)} + 4Ag_{(s)} + 4HNO_{3(aq)}$
$(b)$ $H_3PO_{2(aq)} + 2CuSO_{4(aq)} + 2H_2O_{(l)} \rightarrow H_3PO_{4(aq)} + 2Cu_{(s)} + 2H_2SO_{4(aq)}$
$(c)$ $C_6H_5CHO_{(l)} + 2[Ag(NH_3)_2]^{+}_{(aq)} + 3OH^{-}_{(aq)}$ $\rightarrow C_6H_5COO^{-}_{(aq)} + 2Ag_{(s)} + 4NH_{3(aq)} + 2H_2O_{(l)}$
$(d)$ $C_6H_5CHO_{(l)} + 2Cu^{2+}_{(aq)} + 5OH^{-}_{(aq)} \rightarrow$ No change observed.
What inference do you draw about the behaviour of $Ag^{+}$ and $Cu^{2+}$ from these reactions?

In the reaction $PCl_3 + Cl_2 \to PCl_5$,

In which of the following reactions does $H_2SO_4$ act as an oxidizing agent?

$HNO_2$ acts as which of the following?